High-density acicular hematite particles, non-magnetic undercoat layer and magnetic recording medium

ABSTRACT

High-density acicular hematite particles comprise acicular hematite particles and a coat comprising an oxide of tin or oxides of tin and antimony, formed on at least a part of surfaces of said acicular hematite particles; and have an average major axial diameter of not more than 0.3 μm, a pH value of not less than 8, a soluble sodium salt content of not more than 300 ppm, calculated as Na, and a soluble sulfate content of not more than 150 ppm, calculated as SO 4 . Such high-density acicular hematite particles is suitable as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium using magnetic particles containing iron as a main ingredient, have an excellent dispersibility in vehicle and a low volume resistivity.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to high-density acicular hematiteparticles, a non-magnetic undercoat layer containing the high-densityacicular hematite particles and a magnetic recording medium having thenon-magnetic undercoat layer. More particularly, the present inventionrelates to high-density acicular hematite particles suitable asnon-magnetic particles for a non-magnetic undercoat layer of a magneticrecording medium using magnetic particles containing iron as a mainingredient, which have an excellent dispersibility in binder resin, a pHvalue of not less than 8, a less content of soluble sodium salts andsoluble sulfates, and a high surface conductivity; a non-magneticundercoat layer containing the high-density acicular hematite particlesand suitably used for a magnetic recording medium using magneticparticles containing iron as a main ingredient; and a magnetic recordingmedium having the non-magnetic undercoat layer.

[0002] With a development of miniaturized and lightweight video or audiomagnetic recording and reproducing apparatuses for long-time recording,magnetic recording media such as a magnetic tape and magnetic disk havebeen increasingly and strongly required to have a higher performance,namely, a higher recording density, higher output characteristic, inparticular, an improved frequency characteristic and a lower noiselevel.

[0003] Various attempts have been made at both enhancing the propertiesof magnetic particles and reducing the thickness of a magnetic recordinglayer in order to improve these properties of a magnetic recordingmedium.

[0004] The enhancement of the properties of magnetic particles willfirst be described.

[0005] Magnetic particles are required to have, in order to satisfy theabove-described demands on a magnetic recording medium, properties suchas a high coercive force and a large saturation magnetization.

[0006] As magnetic particles suitable for high-output and high-densityrecording, acicular magnetic particles containing iron as a mainingredient which are obtained by heat-treating acicular goethiteparticles or acicular hematite particles in a reducing gas are widelyknown.

[0007] Acicular magnetic particles containing iron as a main ingredienthave a high coercive force and a large saturation magnetization, sincethe acicular magnetic particles containing iron as a main ingredientused for a magnetic recording medium are very fine particles having aparticle size of not more than 1 μm, particularly, 0.01 to 0.3 μm.Therefore, such particles easily corrode, and the magnetic propertiesthereof are deteriorated, especially, the saturation magnetization andthe coercive force are reduced.

[0008] In order to maintain the characteristics of a magnetic recordingmedium which uses magnetic particles containing iron as a mainingredient as the magnetic particles, over a long period, it is stronglydemanded to suppress the corrosion of the acicular magnetic particlescontaining iron as a main ingredient as much as possible.

[0009] A reduction in the thickness of a magnetic recording layer willnow be described.

[0010] Video tapes have recently been required more and more to have ahigher picture quality, and the frequencies of carrier signals recordedin recent video tapes are higher than those recorded in conventionalvideo tapes. In other words, the signals in the short-wave region havecome to be used, and as a result, the magnetization depth from thesurface of a magnetic tape has come to be remarkably small.

[0011] With respect to short wavelength signals, a reduction in thethickness of a magnetic recording layer is also strongly demanded inorder to improve the high output characteristics, especially, an S/Nratio of a magnetic recording medium. This fact is described, forexample, on page 312 of Development of Magnetic Materials and Techniquefor High Dispersion of Magnetic Powder, published by Sogo Gijutsu CenterCo., Ltd. (1982), “. . . the conditions for high-density recording in acoated-layer type tape are that the noise level is low with respect tosignals having a short wavelength and that the high outputcharacteristics are maintained. To satisfy these conditions, it isnecessary that the tape has large coercive force Hc and residualmagnetization Br, and the coating film has a smaller thickness. . . . ”.

[0012] Development of a reduction in the thickness of a magneticrecording layer has caused some problems.

[0013] Firstly, it is necessary to make a magnetic recording layersmooth and to eliminate the non-uniformity of thickness. As well known,in order to obtain a smooth magnetic recording layer having a uniformthickness, the surface of the substrate must also be smooth. This factis described on pages 180 and 181 of Materials for SyntheticTechnology—Causes of Friction and Abrasion of Magnetic Tape and HeadRunning System and Measures for Solving the Problem (hereinunderreferred to as “Materials for Synthetic Technology” (1987), published bythe Publishing Department of Technology Information Center, “ . . . thesurface roughness of a hardened magnetic layer depends on the surfaceroughness of the substrate (back surface roughness) so largely as to beapproximately proportional, . . . , since the magnetic layer is formedon the substrate, the more smooth the surface of the substrate is, themore uniform and larger head output is obtained, and the more the S/Nratio is improved.”

[0014] Secondly, there has been caused a problem in the strength of anon-magnetic substrate such as a base film with a tendency of thereduction in the thickness of the non-magnetic substrate in response tothe demand for a thinner magnetic layer. This fact is described, forexample, on page 77 of the above-described Development of MagneticMaterials and Technique for High Dispersion of Magnetic Powder, “ . . .Higher recording density is a large problem assigned to the presentmagnetic tape. This is important in order to shorten the length of thetape and to miniaturize the size of a cassette and to enable long-timerecording. For this purpose, it is necessary to reduce the thickness ofa substrate . . . . With the tendency of reduction in the filmthickness, the stiffness of the tape also reduces to such an extent asto make smooth travel in a recorder difficult. Therefore, improvement ofthe stiffness of a video tape both in the machine direction and in thetransverse direction is now strongly demanded. . . . ”

[0015] The end portion of a magnetic recording medium such as a magnetictape, especially, a video tape is judged by detecting a portion of themagnetic recording medium at which the light transmittance is large by avideo deck. If the light transmittance of the whole part of a magneticrecording layer is made large by the thinner magnetic recording mediumor the ultrafine magnetic particles dispersed in the magnetic recordinglayer, it is difficult to detect the portion having a large lighttransmittance by a video deck. For reducing the light transmittance ofthe whole part of a magnetic recording layer, carbon black or the likeis added to the magnetic recording layer. It is, therefore, essential toadd carbon black or the like to a magnetic recording layer in thepresent video tapes.

[0016] However, addition of a large amount of non-magnetic particlessuch as carbon black impairs not only the enhancement of the recordingdensity but also the development of a thinner recording layer. Thereforein order to reduce the magnetization depth from the surface of themagnetic tape and to produce a thinner magnetic recording layer, it isstrongly demanded to reduce, as much as possible, the quantity ofnon-magnetic particles such as carbon black which are added to amagnetic recording layer.

[0017] It is also strongly demanded that the light transmittance of amagnetic recording layer should be small even if the carbon black or thelike which is added to the magnetic recording layer is reduced to asmall amount. From this point of view, improvements in a substrate arenow in strong demand.

[0018] Further, in order to reduce not only the above-mentioned opticaltransmittance but also surface resistivity of the magnetic recordingmedium, carbon black has been conventionally added to a magneticrecording layer thereof.

[0019] The use of carbon black in the magnetic recording medium isdescribed in more detail below.

[0020] In the case where the magnetic recording medium has a highsurface resistivity, the electrostatically charged amount on themagnetic recording medium is increased, so that cutting wastes ofmagnetic recording media or dusts are attached to the surface ofmagnetic recording medium upon production or use of the magneticrecording medium, thereby increasing occurrence of drop-out.

[0021] Consequently, in order to lower the surface resistivity of themagnetic recording medium to about 10⁸ Ωcm, a conductive compound suchas carbon black has been generally added to a magnetic recording layerthereof in an amount of not less than about 5 parts by weight based on100 parts by weight of magnetic particles used therein.

[0022] However, such an increase in amount of non-magnetic substancesuch as carbon black in the magnetic recording layer tends to cause thedeterioration in signal recording property and inhibit the reduction inthickness of the magnetic recording layer.

[0023] Various efforts have been made to improve a base film for amagnetic recording layer with a demand for a thinner magnetic recordinglayer and a thinner non-magnetic substrate. A magnetic recording mediumhaving at least one undercoat layer (hereinunder referred to“non-magnetic undercoat layer”) comprising a binder resin andnon-magnetic particles containing iron as a main ingredient such ashematite particles which are dispersed therein, on a non-magneticsubstrate such as a base film has been proposed and put to practical use(Japanese Patent Publication (KOKOKU) No. 6-93297 (1994), JapanesePatent Application Laid-Open (KOKAI) Nos. 62-159338 (1987), 63-187418(1988), 4-167225 (1992), 4-325915 (1992), 5-73882 (1993), 5-182177(1993), 5-347017 (1993), 6-60362 (1994), 9-35245 (1997), etc.)

[0024] Further, various attempts for reducing the content of carbonblack in the magnetic recording layer and lowering the surfaceresistivity of the magnetic recording medium as low as possible, havebeen conducted. For example, it is known that the surfaces ofnon-magnetic particles dispersed in the above-mentioned non-magneticundercoat layer are coated with a tin compound or an antimony compound(Japanese Patent Nos. 2566088 and 2566089, Japanese Patent Publication(KOKOKU) No. 5-33446(1993), Japanese Patent Applications Laid-open(KOKAI) Nos. 6-60360(1994), 7-176030(1995), 8-50718(1996),8-203063(1996), 8-255334, 9-27116(1997) or the like).

[0025] For example, Japanese Patent Application Laid-Open (KOKAI) No.5-182177 (1993) discloses a magnetic recording medium comprising: anon-magnetic substrate; a non-magnetic undercoat layer formed on thenon-magnetic substrate and produced by dispersing inorganic particles ina binder resin; and a magnetic layer formed on the non-magneticundercoat layer and produced by dispersing ferromagnetic particles in abinder resin while the non-magnetic undercoat layer is wet; wherein themagnetic layer has a thickness of not more than 1.0 μm in a dried state,the non-magnetic undercoat layer contains non-magnetic inorganicparticles with surface layers coated with an inorganic oxide, theinorganic oxide coating the surfaces of the non-magnetic inorganicparticles contained in the non-magnetic undercoat layer is at least oneselected from the group consisting of Al₂O₃, SiO₂ and ZrO₂, and theamount of the inorganic oxide coating the non-magnetic inorganicparticles is 1 to 21 wt % in the case of Al₂O₃, 0.04 to 20 wt % in thecase of SiO₂, and 0.05 to 15 wt % in the case of ZrO₂, base on the totalweigh of the magnetic inorganic particles.

[0026] In Japanese Patent No. 2566088, there is described a magneticrecording medium comprising a non-magnetic substrate, a non-magneticundercoat layer formed on the non-magnetic substrate, comprising abinder resin and non-magnetic inorganic particles dispersed in thebinder resin and coated with at least one oxide selected from the groupconsisting of Al₂O₃, SiO₂, ZrO₂, Sb₂O₃ and ZnO, and a magnetic uppercoatlayer formed on the non-magnetic undercoat layer, comprising a binderresin and ferromagnetic particles dispersed in the binder resin, whereinthe magnetic uppercoat layer has a dry thickness of not more than 1.0μm; the non-magnetic undercoat layer has a dry thickness of 0.5 to 10μm; and the ferromagnetic particles have a major axial diameter of notmore than 0.3 μm.

[0027] At present, there has been more demanded non-magnetic particlesfor non-magnetic undercoat layer of a magnetic recording medium, whichare capable of furnishing a non-magnetic undercoat layer havingexcellent surface smoothness and mechanical strength by dispersing thenon-magnetic particles in a binder resin; which are capable offurnishing a magnetic recording layer having a surface smoothness and athin and uniform thickness when the magnetic recording layer is formedon the non-magnetic undercoat layer; which are capable of furnishing amagnetic recording medium having a low transmittance and a low surfaceresistivity; and which are capable of preventing the corrosion ofmagnetic particles containing iron as a main ingredient, which aredispersed in the magnetic recording layer. However, such non-magneticparticles have not been furnished yet.

[0028] That is, it has been reported that the above-mentionedconventional magnetic recording medium using hematite particles asnon-magnetic particles for non-magnetic undercoat layer thereof, areimproved in surface smoothness and mechanical strength of thenon-magnetic undercoat layer; is capable of forming a magnetic recordinglayer having a surface smoothness, and a thin and uniform thickness uponthe formation of the magnetic recording layer; and exhibit a lowtransmittance. However, these properties reported are stillunsatisfactory. Especially, as described in Comparative Exampleshereinafter, the surface resistivity of these conventional magneticrecording medium is as high as 10⁹ to 10¹¹ Ωcm.

[0029] On the other hand, in the case of the magnetic recording mediumhaving the non-magnetic undercoat layer containing non-magneticparticles coated with a tin compound or an antimony compound anddispersed in a binder resin, the non-magnetic undercoat layer isdeteriorated in surface smoothness and mechanical strength, though thesurface resistivity thereof is low. Accordingly, the magnetic recordinglayer formed on such a non-magnetic undercoat layer necessarily has arough surface and an uneven thickness, and exhibit an unsatisfactorytransmittance.

[0030] Further, there has also been pointed out such a problem that themagnetic particles containing iron as a main ingredient, which aredispersed in the magnetic recording layer, undergo server corrosionafter the production of the magnetic recording medium, thereby causingthe considerable deterioration in magnetic properties thereof.

[0031] As a result of the present inventors' earnest studies for solvingthe above-mentioned problems, it has been found that by coating at leasta part of surfaces of specific acicular hematite particles with an oxideof tin or an oxide of tin and antimony, and controlling the pH value tonot less than 8 and contents of soluble sodium salts and solublesulfates to a certain range, the obtained high-density acicular hematiteparticles exhibit a low surface resistivity and an excellentdispersibility in a vehicle. The present invention has been attained onthe basis of this finding.

SUMMARY OF THE INVENTION

[0032] It is an object of the present invention to provide non-magneticparticles for non-magnetic undercoat layer of a magnetic recordingmedium, which are capable of furnishing a non-magnetic undercoat layerhaving excellent surface smoothness and mechanical strength bydispersing the non-magnetic particles in a binder resin; which arecapable of furnishing a magnetic recording layer having a surfacesmoothness and a thin and uniform thickness upon the formation of themagnetic recording layer; which are capable of furnishing a magneticrecording medium having a low transmittance and a low surfaceresistivity; and which are capable of preventing the corrosion of metalmagnetic particles containing iron as a main ingredient, which aredispersed in the magnetic recording layer.

[0033] It is another object of the present invention to provide anon-magnetic undercoat layer which has excellent surface smoothness andmechanical strength, which is capable of forming thereon a magneticrecording layer, which is capable of imparting an excellent surfacesmoothness, a low transmittance and a low surface resistivity to themagnetic recording layer when formed on the non-magnetic undercoatlayer, and which is capable of preventing metal magnetic particlescontaining iron as a main ingredient, which are dispersed in themagnetic recording layer, from being corroded, thereby inhibiting thedeterioration in magnetic properties thereof.

[0034] It is other object of the present invention to provide a magneticrecording medium which has an excellent surface smoothness, a lowtransmittance and a low surface resistivity, and in which the corrosionof metal magnetic particles containing iron as a main ingredient, whichare dispersed in the magnetic recording layer, is prevented, therebyinhibiting the deterioration in magnetic properties thereof.

[0035] To accomplish the aim, in a first aspect of the presentinvention, there is provided high-density acicular hematite particlescomprising acicular hematite particles and a coat comprising an oxide oftin or oxides of tin and antimony, formed on at least a part of surfacesof the acicular hematite particles; and having an average major axialdiameter of not more than 0.3 μm, a pH value of not less than 8, asoluble sodium salt content of not more than 300 ppm (calculated as Na)and a soluble sulfate content of not more than 150 ppm (calculated asSO₄).

[0036] In a second aspect of the present invention, there is providedhigh-density acicular hematite particles comprising acicular hematiteparticles, a first coat comprising an oxide of tin or oxides of tin andantimony, formed on at least a part of surfaces of the acicular hematiteparticles, and a second coat comprising at least one compound selectedfrom the group consisting of a hydroxide of aluminum, an oxide ofaluminum, a hydroxide of silicon and an oxide of silicon, formed on atleast a part of surfaces of said high-density acicular hematiteparticles; and

[0037] having an average major axial diameter of not more than 0.3 μm, apH value of not less than 8, a soluble sodium salt content of not morethan 300 ppm (calculated as Na) and a soluble sulfate content of notmore than 150 ppm (calculated as SO₄).

[0038] In a third aspect of the present invention, there is provided anon-magnetic undercoat layer comprising the high-density acicularhematite particles set forth in the first or second aspect and a binderresin, formed on a non-magnetic substrate.

[0039] In a fourth aspect of the present invention, there is provided amagnetic recording medium comprising:

[0040] a non-magnetic substrate;

[0041] a non-magnetic undercoat layer comprising the high-densityacicular hematite particles set forth in the first or second aspect anda binder resin, formed on said non-magnetic substrate; and

[0042] a magnetic recording layer comprising magnetic particlescontaining iron as a main ingredient and a binder resin, formed on saidnon-magnetic undercoat layer.

[0043] In a fifth aspect of the present invention, there is provided aprocess for producing high-density acicular hematite particles set forthin claim 1, comprising:

[0044] heat-dehydrating acicular goethite particles coated with ahydroxide of tin to obtain low-density acicular hematite particles;

[0045] heat-treating said low-density acicular hematite particles at atemperature of not less than 550° C. to obtain high-density acicularhematite particles coated with an oxide of tin;

[0046] wet-pulverizing a slurry containing said high-density acicularhematite particles;

[0047] adjusting the pH value of said slurry to not less than 13;

[0048] heat-treating said slurry at a temperature of not less than 80°C.; and

[0049] filtering said slurry to separate high-density acicular hematiteparticles therefrom, followed by washing with water and drying.

[0050] In a sixth aspect of the present invention, there is provided aprocess for producing high-density acicular hematite particles set forthin claim 1, comprising:

[0051] wet-pulverizing a slurry containing high-density acicularhematite particles obtained by heat-treating at a temperature of notless than 550° C. low-density acicular hematite particles produced byheat-dehydrating acicular goethite particles coated with a sinteringpreventive agent;

[0052] adjusting the pH value of said slurry to not less than 13;

[0053] heat-treating said slurry at a temperature of not less than 80°C.; and

[0054] filtering said slurry to separate high-density acicular hematiteparticles therefrom, followed by washing with water and drying;

[0055] treating the obtained high-density acicular hematite particleswith an aqueous solution containing a tin compound to obtainhigh-density acicular hematite particles coated with a hydroxide of tin;and

[0056] heat-treating said high-density acicular hematite particlescoated with a hydroxide of tin at a temperature of not less than 300° C.

[0057] In a seventh aspect of the present invention, there is provided aprocess for producing high-density acicular hematite particles set forthin claim 1, comprising:

[0058] heat-dehydrating acicular goethite particles coated withhydroxides of tin and antimony to obtain low-density acicular hematiteparticles;

[0059] heat-treating said low-density acicular hematite particles at atemperature of not less than 550° C. to obtain high-density acicularhematite particles coated with oxides of tin and antimony;

[0060] wet-pulverizing a slurry containing said high-density acicularhematite particles;

[0061] adjusting the pH value of said slurry to not less than 13;

[0062] heat-treating said slurry at a temperature of not less than 80°C.; and

[0063] filtering said slurry to separate high-density acicular hematiteparticles therefrom, followed by washing with water and drying.

[0064] In an eighth aspect of the present invention, there is provided aprocess for producing high-density acicular hematite particles set forthin claim 1, comprising:

[0065] wet-pulverizing a slurry containing high-density acicularhematite particles obtained by heat-treating at a temperature of notless than 550° C. low-density acicular hematite particles produced byheat-dehydrating acicular goethite particles coated with a sinteringpreventive agent;

[0066] adjusting the pH value of said slurry to not less than 13;

[0067] heat-treating said slurry at a temperature of not less than 80°C.; and

[0068] filtering said slurry to separate high-density acicular hematiteparticles therefrom, followed by washing with water and drying;

[0069] treating the obtained high-density acicular hematite particleswith an aqueous solution containing a tin compound and an antimonycompound to obtain high-density acicular hematite particles coated withhydroxides of tin and antimony; and

[0070] heat-treating said high-density acicular hematite particlescoated with hydroxides of tin and antimony at a temperature of not lessthan 300° C.

DETAILED DESCRIPTION OF THE INVENTION

[0071] The present invention is described in detail below.

[0072] First, the high-density acicular hematite particles in which atleast a part of the surface thereof is coated with an oxide of tin or anoxide of tin and antimony, are described.

[0073] The amount of the oxide of tin coated on surface of the particlesis usually 0.5 to 500% by weight (calculated as Sn) based on the weightof the acicular hematite particles. When the amount of the oxide of tinis less than 0.5% by weight, the surface of the particles cannot besatisfactorily coated with the oxide of tin as a conductive substance,so that it becomes impossible to attain a sufficient effect of reducinga surface resistivity of the magnetic recording medium. On the otherhand, when the amount of the oxide of tin is more than 500% by weight,although a sufficient effect of reducing a surface resistivity of themagnetic recording medium can be obtained, the effect is alreadysaturated and, therefore, the use of such an excessive amount of theoxide of tin is meaningless. In view of the surface resistivity of theobtained magnetic recording medium and economy of the productionthereof, the amount of the oxide of tin is preferably 1.0 to 250% byweight, more preferably 2.0 to 200% by weight (calculated as Sn) basedon the weight of the acicular hematite particles.

[0074] The amount of the oxide of antimony coated on surfaces of theparticles is usually not more than 50% by weight, preferably 0.05 to 50%by weight (calculated as Sb) based on the weight of the acicularhematite particles. When the amount of the oxide of antimony is morethan 50% by weight, although a sufficient effect of reducing a surfaceresistivity of the magnetic recording medium can be obtained, the effectis already saturated and, therefore, the use of such an excessive amountof the oxide of antimony is meaningless. In view of the surfaceresistivity of the obtained magnetic recording medium and economy of theproduction thereof, the amount of the oxide of antimony is morepreferably 0.1 to 25% by weight (calculated as Sb) based on the weightof the acicular hematite particles.

[0075] In the case where the surface of the particles are coated withthe oxide of tin and antimony, the weight ratio of tin to antimony isusually 20:1 to 1:1, preferably 15:1 to 2:1. When the amount of tin isless than that of antimony, it may become difficult to effectivelyreduce a surface resistivity of the magnetic recording medium. When theweight ratio of tin to antimony exceeds 20, it may become difficult tomore effectively reduce a surface resistivity of the magnetic recordingmedium, because the amount of tin is too small.

[0076] The high-density acicular hematite particles coated with theoxide of tin or the oxides of tin and antimony according to the presentinvention have an average major axial diameter of not more than 0.3 μm,a pH value of not less than 8, a soluble sodium salt content of not morethan 300 ppm (calculated as Na) and a soluble sulfate content of notmore than 150 ppm (calculated as SO₄).

[0077] The high-density acicular hematite particles in the presentinvention have an aspect ratio (average major axial diameter/averageminor axial diameter) (hereinunder referred to merely as “aspect ratio”)of not less than 2:1, preferably not less than 3:1. The upper limit ofthe aspect ratio is usually 20:1, preferably 10:1 with the considerationof the dispersibility in the vehicle. The shape of the acicularparticles here may have not only acicular but also spindle-shaped, riceball-shaped or the like.

[0078] When the aspect ratio is less than 2:1, it is difficult to obtaina desired film strength of the magnetic recording medium.

[0079] The average major axial diameter of the high-density acicularhematite particles of the present invention is not more than 0.3 μm,preferably 0.005 to 0.3 μm. When the average major axial diameterexceeds 0.3 μm, the particle size is so large as to impair the surfacesmoothness. With the consideration of the dispersibility in the vehicleand the surface smoothness of the coated film, the more preferableaverage major axial diameter is 0.02 to 0.2 μm.

[0080] The average minor axial diameter of the high-density acicularhematite particles of the present invention is usually 0.0025 to 0.15μm. When the average minor axial diameter is less than 0.0025 μm,dispersion in the vehicle may be unfavorably difficult. On the otherhand, when the average minor axial diameter exceeds 0.15 μm, theparticle size may be apt to become so large as to impair the surfacesmoothness. With the consideration of the dispersibility in the vehicleand the surface smoothness of the coated film, the more preferableaverage minor axial diameter is 0.01 to 0.10 μm.

[0081] The BET specific surface area of the high-density acicularhematite particle of the present invention is usually not less than 35m²/g. When it is less than 35 m²/g, the acicular hematite particles maybe coarse or sintering may be sometimes caused between particles, whichare apt to exert a deleterious influence on the surface smoothness ofthe coated film. The BET surface area thereof is more preferably notless than 40 m²/g, even more preferably not less than 45 m²/g, and theupper limit thereof is usually 150 m²/g. The upper limit is preferably100 m²/g, more preferably 80 m²/g with the consideration of thedispersibility in the vehicle.

[0082] The degree of densification (S_(BET)/S_(TEM)) of hematiteparticles is represented by the ratio of the specific surface area(S_(BET)) measured by a BET method and the surface area (S_(TEM))calculated from the major axial diameter and the minor axial diameterwhich were measured from the particles in an electron micrograph.

[0083] The S_(BET)/S_(TEM) value of hematite particles according to thepresent invention is usually 0.5 to 2.5. When the S_(BET)/S_(TEM) valueis less than 0.5, although the hematite particles have been densified,the particles may adhere to each other due to sintering therebetween,and the particle size may increase, so that a sufficient surfacesmoothness of the coated film may be not obtained. On the other hand,when the S_(BET)/S_(TEM) value exceeds 2.5, there may be many pores inthe surfaces of particles and the dispersibility in the vehicle maybecome insufficient. In consideration of the surface smoothness of thecoated film and the dispersibility in the vehicle, the S_(BET)/S_(TEM)value is preferably 0.7 to 2.0, more preferably 0.8 to 1.6.

[0084] The major axial diameter distribution of the high-densityacicular hematite particles of the present invention is preferably notmore than 1.50 in geometrical standard deviation. When it exceeds 1.50,the coarse particles existent sometimes exert a deleterious influence onthe surface smoothness of the coated film. The major axial diameterdistribution is more preferably not more than 1.40, even more preferablynot more than 1.35 in geometrical standard deviation with theconsideration of the surface smoothness of the coated film. From thepoint of view of industrial productivity, the major axial diameterdistribution of the high-density acicular hematite particles obtained isusually 1.01 in geometrical standard deviation.

[0085] The pH value of the high-density acicular hematite particles ofthe present invention is not less than 8. When it is less than 8, themagnetic particles containing iron as a main ingredient contained in themagnetic recording layer formed on the non-magnetic undercoat layer aregradually corroded, thereby causing a deterioration in the magneticproperties. With the consideration of a corrosion preventive effect onthe magnetic particles containing iron as a main ingredient, the pHvalue of the particles is preferably not less than 8.5, more preferablynot less than 9.0. The upper limit is usually 12, preferably 11, morepreferably 10.5.

[0086] The content of soluble sodium salts in the high-density acicularhematite particles of the present invention is not more than 300 ppmsoluble sodium (calculated as Na). When it exceeds 300 ppm, the magneticparticles containing iron as a main ingredient contained in the magneticrecording layer formed on the non-magnetic undercoat layer are graduallycorroded, thereby causing a deterioration in the magnetic properties. Inaddition, the dispersion property of the high-density acicular hematiteparticles in the vehicle is easily impaired, and the preservation of themagnetic recording medium is deteriorated and efflorescence is sometimescaused in a highly humid environment. With the consideration of acorrosion preventive effect on the magnetic particles containing iron asa main ingredient, the content of soluble sodium salt is preferably notmore than 250 ppm, more preferably not more than 200 ppm, even morepreferably not more than 150 ppm. From the point of view of industrysuch as productivity, the lower limit thereof is preferably about 0.01ppm.

[0087] The content of soluble sulfate in the high-density acicularhematite particles of the present invention is not more than 150 ppmsoluble sulfate (calculated as SO₄). When it exceeds 150 ppm, themagnetic particles containing iron as a main ingredient contained in themagnetic recording layer formed on the non-magnetic undercoat layer aregradually corroded, thereby causing a deterioration in the magneticproperties. In addition, the dispersion property of the high-densityacicular hematite particles in the vehicle is easily impaired, and thepreservation of the magnetic recording medium is deteriorated andefflorescence is sometimes caused in a highly humid environment. Withthe consideration of a corrosion preventive effect on the magneticparticles containing iron as a main ingredient, the content of solublesulfate is preferably not more than 70 ppm, more preferably not morethan 50 ppm. From the point of view of industry such as productivity,the lower limit thereof is preferably about 0.01 ppm.

[0088] The high-density acicular hematite particles according to thepresent invention in which at least a part of the surface thereof iscoated with the oxide of tin or the oxide of tin and antimony, have avolume resistivity of 10³ to 5× 10⁷ Ωcm. When the volume resistivity ofacicular hematite particles is more than 10⁸ Ωcm, it may becomedifficult to obtain a magnetic recording medium having a sufficientlylow surface resistivity.

[0089] At least a part of the surfaces of the high-density acicularhematite particles coated of the present invention may be coated with atleast one selected from the group consisting of a hydroxide of aluminum,an oxide of aluminum, a hydroxide of silicon and an oxide of silicon.When the acicular hematite particles coated with the above-describedcoating material are dispersed in a vehicle, they have an affinity withthe binder resin and it is easy to obtain a desired dispersibility.

[0090] The amount of aluminum hydroxide, aluminum oxide, siliconhydroxide or silicon oxide used as the coating material is usually notless than 50 wt %, preferably 0.01 to 50 wt % (calculated as Al orSiO₂). When it is less than 0.01 wt %, the dispersibility improvingeffect may be insufficient. When the amount exceeds 50 wt %, the coatingeffect becomes saturated, so that it is meaningless to add a coatingmaterial more than necessary. From the point of view of dispersibilityin the vehicle, the preferable amount of coating material is preferably0.05 to 20 wt % (calculated as Al or SiO₂).

[0091] Various properties of the high-density acicular hematiteparticles coated with a coating material of the present invention, suchas aspect ratio, average major axial diameter, average minor axialdiameter, pH value, the content of soluble sodium salt, content ofsoluble sulfate, BET specific surface area, major axial diameterdistribution, degree of densification, and volume resistivity areapproximately equivalent in values to those of the high-density acicularhematite particles of the present invention the surfaces of which arenot coated with a coating material.

[0092] A non-magnetic undercoat layer and a magnetic recording mediumaccording to the present invention will now be explained.

[0093] The magnetic medium of according to the present inventioncomprises a non-magnetic substrate, a non-magnetic undercoat layer and amagnetic recording layer.

[0094] The non-magnetic undercoat layer of the present invention isproduced by forming a coating film on the non-magnetic substrate anddrying the coating film. The non-magnetic coating film is formed byapplying to the surface of the non-magnetic substrate a non-magneticcoating composition which contains the high-density acicular hematiteparticles, a binder resin and a solvent.

[0095] As the non-magnetic substrate, the following materials which areat present generally used for the production of a magnetic recordingmedium are usable as a raw material: a synthetic resin such aspolyethylene terephthalate, polyethylene, polypropylene, polycarbonate,polyethylene naphthalate, polyamide, polyamideimide and polyimide; foiland plate of a metal such as aluminum and stainless steel; and variouskinds of paper. The thickness of the non-magnetic substrate variesdepending upon the material, but it is usually about 1.0 to 300 μm,preferably 2.0 to 200 μm. In the case of a magnetic disc, polyethyleneterephthalate is ordinarily used as the non-magnetic substrate. Thethickness thereof is usually 50 to 300 μm, preferably 60 to 200 μm. Inthe case of a magnetic tape, when polyethylene terephthalate is used asthe non-magnetic substrate, the thickness thereof is usually 3 to 100μm, preferably 4 to 20 μm. When polyethylene naphthalate is used, thethickness thereof is usually 3 to 50 μm, preferably 4 to 20 μm. Whenpolyamide is used, the thickness thereof is usually 2 to 10 μm,preferably 3 to 7 μm.

[0096] The thickness of the non-magnetic undercoat layer obtained bycoating the non-magnetic substrate with a coating composition and dryingthe coating film, is usually 0.2 to 10.0 μm, preferably 0.5 to 5.0 μm.When the thickness is less than 0.2 μm, not only it is impossible toameliorate the surface roughness of the non-magnetic substrate but alsothe strength is insufficient.

[0097] As the binder resin in the present invention, the followingresins which are at present generally used for the production of amagnetic recording medium are usable: vinyl chloride-vinyl acetatecopolymer, urethane resin, vinyl chloride-vinyl acetate-maleic acidcopolymer, urethane elastomer, butadiene-acrylonitrile copolymer,polyvinyl butyral, cellulose derivative such as nitrocellulose,polyester resin, synthetic rubber resin such as polybutadiene, epoxyresin, polyamide resin, polyisocyanate, electron radiation curing acrylurethane resin and mixtures thereof. Each of these resin binders maycontain a functional group such as —OH, —COOH, —SO₃M, —OPO₂M₂ and —NH₂,wherein M represents H, Na or K. With the consideration of thedispersibility of the particles, a binder resin containing a functionalgroup —COOH or —SO₃M is preferable.

[0098] The mixing ratio of the high-density acicular hematite particleswith the binder resin is usually 5 to 2000 parts by weight, preferably100 to 1000 parts by weight based on 100 parts by weight of the binderresin.

[0099] It is possible to add a lubricant, a polishing agent, anantistatic agent, etc. which are generally used for the production of amagnetic recording medium to the non-magnetic undercoat layer.

[0100] The gloss of the coated film of the non-magnetic undercoat layercontaining high-density acicular hematite particles according to thepresent invention is usually 180 to 280%, preferably 185 to 280%, morepreferably 187 to 280% and the surface roughness Ra thereof is usually2.0 to 13.0 nm, preferably 2.0 to 11.0 nm, more preferably 2.0 to 10.0nm. The Young's modulus (relative value to a commercially availablevideo tape: AV T-120 produced by Victor Company of Japan, Limited)thereof is usually 115 to 150, preferably 120 to 150, more preferably125 to 150.

[0101] The magnetic recording medium according to the present inventionis produced by forming the non-magnetic undercoat layer formed on thenon-magnetic substrate, forming a coating film on the non-magneticundercoat layer by applying a coating composition containing magneticparticles containing iron as a main ingredient, a binder resin and asolvent, and drying the coating film to obtain a magnetic recordinglayer.

[0102] The magnetic particles containing iron as a main ingredient usedin the present invention comprises iron or iron and at least oneselected from the group consisting of Co, Al, Ni, P, Si, Zn, Ti, Cu, B,Nd, La and Y. Further, the following magnetic particles containing ironas a main ingredient may be exemplified.

[0103] 1) Magnetic particles containing iron as a main ingredientcomprises iron and usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % ofaluminum (calculated as Al) based on the weight of the magneticparticles containing iron as a main ingredient.

[0104] 2) Magnetic particles containing iron as a main ingredientcomprises iron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % ofaluminum (calculated as Al) based on the weight of the magneticparticles containing iron as a main ingredient; and usually 0.05 to 40wt %, preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % of cobalt(calculated as Co) based on the weight of the magnetic particlescontaining iron as a main ingredient.

[0105] 3) Magnetic particles containing iron as a main ingredientcomprises iron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % ofaluminum (calculated as Al) based on the weight of the magneticparticles containing iron as a main ingredient; and usually 0.05 to 10wt %, preferably 0.1 to 7 wt % of at least one selected from the groupconsisting of Nd, La and Y (calculated as the corresponding element)based on the weight of the magnetic particles containing iron as a mainingredient.

[0106] 4) Magnetic particles containing iron as a main ingredientcomprises iron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % ofaluminum (calculated as Al) based on the weight of the magneticparticles containing iron as a main ingredient; usually 0.05 to 40 wt %,preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % of cobalt(calculated as Co) based on the weight of the magnetic particlescontaining iron as a main ingredient; and usually 0.05 to 10 wt %,preferably 0.1 to 7 wt % of at least one selected from the groupconsisting of Nd, La and Y (calculated as the corresponding element)based on the weight of the magnetic particles containing iron as a mainingredient.

[0107] 5) Magnetic particles containing iron as a main ingredientcomprises iron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % ofaluminum (calculated as Al) based on the weight of the magneticparticles containing iron as a main ingredient; and usually 0.05 to 10wt %, preferably 0.1 to 7 wt % of at least one selected from the groupconsisting of Ni, P, Si, Zn, Ti, Cu and B (calculated as thecorresponding element) based on the weight of the magnetic particlescontaining iron as a main ingredient.

[0108] 6) Magnetic particles containing iron as a main ingredientcomprises iron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % ofaluminum (calculated as Al) based on the weight of the magneticparticles containing iron as a main ingredient; usually 0.05 to 40 wt %,preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % of cobalt(calculated as Co) based on the weight of the magnetic particlescontaining iron as a main ingredient; and usually 0.05 to 10 wt %,preferably 0.1 to 7 wt % of at least one selected from the groupconsisting of Ni, P, Si, Zn, Ti, Cu and B (calculated as thecorresponding element) based on the weight of the magnetic particlescontaining iron as a main ingredient.

[0109] 7) Magnetic particles containing iron as a main ingredientcomprises iron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % ofaluminum (calculated as Al) based on the weight of the magneticparticles containing iron as a main ingredient; usually 0.05 to 10 wt %,preferably 0.1 to 7 wt % of at least one selected from the groupconsisting of Nd, La and Y (calculated as the corresponding element)based on the weight of the magnetic particles containing iron as a mainingredient; and usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % of atleast one selected from the group consisting of Ni, P, Si, Zn, Ti, Cuand B (calculated as the corresponding element) based on the weight ofthe magnetic particles containing iron as a main ingredient.

[0110] 8) Magnetic particles containing iron as a main ingredientcomprises iron; usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % ofaluminum (calculated as Al) based on the weight of the magneticparticles containing iron as a main ingredient; usually 0.05 to 40 wt %,preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % of cobalt(calculated as Co) based on the weight of the magnetic particlescontaining iron as a main ingredient; usually 0.05 to 10 wt %,preferably 0.1 to 7 wt % of at least one selected from the groupconsisting of Nd, La and Y (calculated as the corresponding element)based on the weight of the magnetic particles containing iron as a mainingredient; and usually 0.05 to 10 wt %, preferably 0.1 to 7 wt % of atleast one selected from the group consisting of Ni, P, Si, Zn, Ti, Cuand B (calculated as the corresponding element) based on the weight ofthe magnetic particles containing iron as a main ingredient.

[0111] The iron content in the particles is the balance, and ispreferably 50 to 99 wt %, more preferably 60 to 95 wt % (calculated asFe) based on the weight of the magnetic particles containing iron as amain ingredient.

[0112] The magnetic particles containing iron as a main ingredientcomprising (i) iron and Al; (ii) iron, Al and Co, (iii) iron, Al and atleast one rare-earth metal such as Nd, La and Y, or (iv) iron, Al, Coand at least one rare-earth metal such as Nd, La and Y, are preferablefrom the point of the durability of the magnetic recording medium.Further, the magnetic particles containing iron as a main ingredientcomprising iron, Al and at least one rare-earth metal such as Nd, La andY, are more preferable.

[0113] The acicular magnetic particles containing iron as a mainingredient used in the present invention have an average major axialdiameter of usually 0.01 to 0.50 μm, preferably 0.03 to 0.30 μm, morepreferably 0.03 to 0.25 μm, an average minor axial diameter of usually0.0007 to 0.17 μm, preferably 0.003 to 0.10 μm, and an aspect ratio ofusually not less than 3:1, preferably and not less than 5:1. The upperlimit of the aspect ratio is usually 15:1, preferably 10:1 with theconsideration of the dispersibility in the vehicle. The shape of theacicular magnetic particles containing iron as a main ingredient mayhave not only acicular but also a spindle-shaped, rice ball-shaped orthe like.

[0114] As to the magnetic properties of the acicular magnetic particlescontaining iron as a main ingredient used in the present invention, thecoercive force is preferably 1200 to 3200 Oe, more preferably 1500 to3200 Oe, and the saturation magnetization is preferably 100 to 170emu/g, more preferably 130 to 170 emu/g with the consideration of theproperties such as high-density recording.

[0115] As the binder resin for the magnetic recording layer, the samebinder resin as that used for the production of the non-magneticundercoat layer is usable.

[0116] The thickness of the magnetic recording layer obtained byapplying the magnetic coating composition to the non-magnetic undercoatlayer and dried, is ordinarily in the range of 0.01 to 5.0 μm. When thethickness is less than 0.01 μm, uniform coating may be difficult, sothat unfavorable phenomenon such as unevenness on the coating surface isobserved. On the other hand, when the thickness exceeds 5.0 μm, it maybe difficult to obtain desired signal recording property due to aninfluence of diamagnetism. The preferable thickness is in the range of0.05 to 1.0 μm.

[0117] The mixing ratio of the acicular magnetic particles containingiron as a main ingredient with the binder resin in the magneticrecording layer is usually 200 to 2000 parts by weight, preferably 300to 1500 parts by weight based on 100 parts by weight of the binderresin.

[0118] It is possible to add a lubricant, a polishing agent, anantistatic agent, etc. which are generally used for the production of amagnetic recording medium to the magnetic recording layer.

[0119] The magnetic recording medium according to the present inventionhas a coercive force of usually 900 to 3500 Oe, preferably 1000 to 3500Oe, more preferably 1500 to 3500 Oe; a squareness (residual magneticflux density Br/saturation magnetic flux density Bm) of usually 0.85 to0.95, preferably 0.87 to 0.95; a gloss (of the coating film) of usually195 to 300%, preferably 200 to 300%; a surface roughness Ra (of thecoating film) of usually not more than 11.0 nm, preferably 1.0 to 10.0nm, more preferably 1.0 to 9.0 nm; a Young's modulus (relative value toa commercially available video tape: AV T-120 produced by Victor Companyof Japan, Limited) of usually not less than 125, preferably not lessthan 130; a linear adsorption coefficient (of the coating film) ofusually 1.10 to 2.00 μm⁻¹, preferably 1.20 to 2.00 μm⁻¹; and a surfaceresistivity (of the coating film) of usually 10⁴ to 5×10⁸ Ω/sq,preferably 10⁴ to 4.5× 10⁸ Ω/sq, more preferably 10⁴ to 4×10⁸ Ω/sq.

[0120] The corrosiveness represented by a percentage (%) of change inthe coercive force is usually not more than 10.0%, preferably not morethan 9.5%, and the corrosiveness represented by a percentage (%) ofchange in the saturation magnetic flux density Bm is usually not morethan 10.0%, preferably not more than 9.5%.

[0121] Next, the process for producing the high-density acicularhematite particles coated with an oxide of tin or both an oxide of tinand an oxide of antimony according to the present invention, isdescribed below.

[0122] As a starting material for the acicular hematite particles, theremay be used acicular goethite particles.

[0123] In order to produce the high-density acicular hematite particlesof the present invention, acicular goethite particles are produced.Acicular goethite particles are produced by an ordinary method:

[0124] (A) a method of oxidizing a suspension having a pH value of notless than 11 and containing colloidal ferrous hydroxide particles whichis obtained by adding not less than an equivalent of an alkali hydroxidesolution to an aqueous ferrous salt solution, by passing anoxygen-containing gas thereinto at a temperature of not higher than 80°C.;

[0125] (B) a method of producing acicular goethite particles byoxidizing a suspension containing FeCO₃ which is obtained by reacting anaqueous ferrous salt solution with an aqueous alkali carbonate solution,by passing an oxygen-containing gas thereinto after aging, if necessary,the suspension;

[0126] (C) a method of growing acicular seed goethite particles byoxidizing a ferrous hydroxide solution containing colloidal ferroushydroxide particles which is obtained by adding less than an equivalentof an alkali hydroxide solution or an alkali carbonate solution to anaqueous ferrous salt solution, by passing an oxygen-containing gasthereinto, thereby producing acicular seed goethite particles, addingnot less than an equivalent of an alkali hydroxide solution to the Fe²⁺in the aqueous ferrous salt solution, to the aqueous ferrous saltsolution containing the acicular goethite seed particles, and passing anoxygen-containing gas into the aqueous ferrous salt solution; and

[0127] (D) a method of growing acicular seed goethite particles byoxidizing a ferrous hydroxide solution containing colloidal ferroushydroxide particles which is obtained by adding less than an equivalentof an alkali hydroxide solution or an alkali carbonate solution to anaqueous ferrous salt solution, by passing an oxygen-containing gasthereinto, thereby producing acicular seed goethite particles, andgrowing the obtained acicular seed goethite particles in an acidic orneutral region.

[0128] Elements other than Fe such as Ni, Zn, P, Al and Si, which aregenerally added in order to enhance various properties of the particlessuch as the major axial diameter, the minor axial diameter and theaspect ratio, may be added during the reaction system for producing thegoethite particles.

[0129] The acicular goethite particles obtained have an average majoraxial diameter of usually 0.005 to 0.4 μm, an average minor axialdiameter of usually 0.0025 to 0.20 μm and a BET specific of aboutusually 50 to 250 m²/g, and contain ordinarily soluble sodium salts of300 to 1500 ppm soluble sodium (calculated as Na) and ordinarily solublesulfates of 100 to 3000 ppm soluble sulfate (calculated as SO₄).

[0130] The surfaces of the above-mentioned acicular goethite particlesare then coated with a hydroxide of tin or hydroxides of tin andantimony.

[0131] In the coating-treatment, a tin compound or a tin compound and anantimony compound is added to a water suspension obtained by dispersingthe acicular goethite particles in an aqueous solution. The suspensionis stirred and if required, the pH value of the suspension is adjustedproperly to coat the acicular goethite particles with the hydroxide oftin or the hydroxides of tin and antimony. Then, the suspension is thenfiltered to separate the coated acicular goethite particles therefrom.The coated acicular goethite particles is further washed with water,dried and pulverized.

[0132] As the tin compound added, there may be exemplified alkalistannates such as sodium stannate, tin salts such as stannous chloride,stannic chloride, stannous sulfate, stannic sulfate, stannous nitrate,stannic nitrate, stannous acetate or stannic acetate, or the like. Theamount of the tin compound added is usually 0.5 to 500% by weight,preferably 1 to 250% by weight (calculated as Sn) based on the weight ofthe acicular goethite particles. When the amount of the tin compoundadded is less than 0.5% by weight, the acicular goethite particlescannot be sufficiently coated with the hydroxide of tin. On the otherhand, when the amount of the tin compound added is more than 500% byweight, the effect by the addition is saturated and, therefore, theaddition of such an excessive amount of the tin compound is meaningless.

[0133] As the antimony compound added, there may be exemplified antimonysalts such as antimonous chloride, antimonic chloride or antimonysulfate. The amount of the antimony compound added is usually not morethan 50% by weight, preferably 0.05 to 50% by weight (calculated as Sb)based on the weight of the acicular goethite particles. When the amountof the antimony compound added is more than 50% by weight, the effect bythe addition is saturated and, therefore, the addition of such anexcessive amount of the antimony compound is meaningless.

[0134] The thus obtained acicular goethite particles coated with thehydroxide of tin or the hydroxides of tin and antimony are heated at atemperature as high as not less than 550° C. to produce high-densityacicular hematite particles. Alternatively, the coated acicular goethiteparticles may be heat-dehydrated at a temperature of 250 to 500° C. formlow-density acicular hematite particles, and then, are heat-treated at atemperature as high as not less than 550° C. to produce high-densityacicular hematite particles. In order to obtain the high-densityacicular hematite particles maintaining the shape or configuration oforiginal acicular goethite particles, the latter method is preferred.

[0135] It is preferred to coat the particles with a sintering preventivebefore the heat-treatment at a high temperature in order to obtainhigh-density acicular hematite particles which retain the shapes of theacicular goethite particles. The acicular goethite particles coated witha sintering preventive contain soluble sodium salts of usually 500 to2000 ppm soluble sodium (calculated as Na) and soluble sulfates ofusually 300 to 3000 ppm soluble sulfate (calculated as SO₄), and havethe BET specific surface area of usually about 50 to 250 m²/g. Thecoating-treatment using a sintering preventive is composed of the stepsof: adding a sintering preventive to an aqueous suspension containingthe acicular goethite particles, mixing and stirring the suspension,filtering out the particles, washing the particles with water, anddrying the particles.

[0136] Incidentally, in the case of the acicular goethite particlescoated with the hydroxide of tin or the hydroxides of tin and antimony,the hydroxide of tin or the hydroxides of tin and antimony works on assintering-preventive agent, and therefore, such coated acicular goethiteparticles may further be coated with the sintering-preventive agent.

[0137] The amount of sintering preventive existent on the surfaces ofthe acicular hematite particles of the present invention variesdepending upon various conditions such as the kind of sinteringpreventive, the pH value thereof in an aqueous alkali solution and theheating temperature, it is usually not more than 10 wt %, preferably0.05 to 10 wt % based on the total weight of the particles.

[0138] As the sintering preventive, sintering preventives generally usedare usable. For example, phosphorus compounds such as sodiumhexametaphosphate, polyphospholic acid and orthophosphoric acid; siliconcompounds such as #3 water glass, sodium orthosilicate, sodiummetasilicate and colloidal silica; boron compounds such as boric acid;aluminum compounds including aluminum salts such as aluminum acetate,aluminum sulfate, aluminum chloride and aluminum nitride, alkalialuminate such as sodium aluminate, and alumina sol and aluminumhydroxide; and titanium compounds such as titanyl sulfate may beexemplified.

[0139] The low-density acicular hematite particles obtained byheat-treating the acicular goethite particles coated with a sinteringpreventive at a temperature of 250 to 500° C. have an average majoraxial diameter of usually 0.005 to 0.30 μm, an average minor axialdiameter of usually 0.0025 to 0.15 μm, a BET specific surface area ofusually about 70 to 350 m²/g and contain soluble sodium salts of usually500 to 2000 ppm soluble sodium (calculated as Na) and soluble sulfatesof usually 300 to 4000 ppm soluble sulfate (calculated as SO₄). When thetemperature for heat-treating the goethite particles is less than 250°C., the dehydration reaction takes a long time. On the other hand, Whenthe temperature exceeds 500° C., the dehydration reaction is abruptlybrought out, so that it is difficult to retain the shapes because thesintering between particles is caused. The low-density acicular hematiteparticles obtained by heat-treating the goethite particles at a lowtemperature are low-density particles having a large number ofdehydration pores through which H₂O is removed from the goethiteparticles and the BET specific surface area thereof is about 1.2 to 2times larger than that of the acicular goethite particles as thestarting material.

[0140] The low-density hematite particles are then heat-treated at atemperature of not less than 550° C. to obtain a high-density acicularhematite particles. The upper limit of the heating temperature ispreferably 850° C. The high-density hematite particles contain solublesodium salts of usually 500 to 4000 ppm soluble sodium (calculated asNa) and soluble sulfates of usually 300 to 5000 ppm soluble sulfate(calculated as SO₄), and the BET specific surface area thereof isusually about 35 to 150 m²/g.

[0141] When the heat-treating temperature is less than 550° C., sincethe densification is insufficient, a large number of dehydration poresexist within and on the surface of the hematite particles, so that thedispersion in the vehicle is insufficient. Further, when thenon-magnetic undercoat layer is formed from these particles, it isdifficult to obtain a coated film having a smooth surface. On the otherhand, when the temperature exceeds 850° C., although the densificationof the hematite particles is sufficient, since sintering is caused onand between particles, the particle size increases, so that it isdifficult to obtain a coated film having a smooth surface.

[0142] The obtained acicular hematite particles are pulverized by adry-process, and formed into a slurry. The obtained slurry is thenpulverized by a wet-process so as to deagglomerate coarse particles. Inthe wet-pulverization, ball mill, sand grinder, colloid mill or the likeis used and wet-pulverization is conducted until coarse particles havinga particle size of at least 44 μm are substantially removed. That is,the wet-pulverization is carried out until the amount of the coarseparticles having a particle size of not less than 44 μm becomes tousually not more than 10% by weight, preferably not more than 5% byweight, more preferably 0% by weight based on the total weight of theparticles. When the amount of the coarse particles having a particlesize of not less than 44 μm is more than 10% by weight, the effect oftreating the particles in an aqueous alkali solution at the next step isnot attained.

[0143] The acicular hematite particles with coarse particles removedtherefrom are heat-treated in a slurry at a temperature of usually notless than 80° C. after the pH value of the slurry is adjusted to notless than 13 by adding an aqueous alkali solution such as sodiumhydroxide.

[0144] The concentration of the alkali suspension containing theacicular hematite particles and having a pH value of not less than 13 ispreferably 50 to 250 g/liter.

[0145] When the pH value of the alkali suspension containing theacicular hematite particles is less than 13, it is impossible toeffectively remove the solid crosslinking caused by the sinteringpreventive which exists on the surfaces of the hematite particles, sothat it is impossible to wash out the soluble sodium slat, solublesulfate, etc. existing within and on the surfaces of the particles. Theupper limit of the pH value is usually about 14. When the effect ofremoving the solid crosslinking caused by the sintering preventive whichexists on the surfaces of the hematite particles, the effect of washingout the soluble sodium slat, soluble sulfate, etc., and the effect ofremoving the alkali which adheres to the surfaces of hematite particlesin the process of the heat-treatment of the aqueous alkali suspensionare taken into consideration, the preferable pH value thereof is in therange of 13.1 to 13.8.

[0146] The heat-treating temperature in the aqueous alkali suspensionwhich contains the acicular hematite particles and has a pH value of notless than 13, is usually not less than 80° C., preferably not less than90° C. If the temperature is less than 80° C., it is difficult toeffectively remove the solid crosslinking caused by the sinteringpreventive which exists on the surfaces of the hematite particles. Theupper limit of the heating temperature is preferably 103° C., morepreferably 100° C. When the heating temperature exceeds 103° C.,although it is possible to effectively remove the solid crosslinking,since an autoclave or the like is necessary or solution boils under anormal pressure, it is not advantageous from the point of view ofindustry.

[0147] The acicular hematite particles heat-treated in the aqueousalkali suspension are, thereafter, filtered out and washed with water byan ordinary method so as to remove the soluble sodium salt and solublesulfate which are washed out of the interiors and the surfaces of theparticles and the alkali such as sodium or the like adhered to thesurfaces of the hematite particles in the process of heat-treatment withthe aqueous alkali suspension, and then dried.

[0148] As the method of washing the particles with water, a methodgenerally industrially used such as a decantation method, a dilutionmethod using a filter thickener and a method of passing water into afilter press is adopted.

[0149] If the soluble sodium salt and soluble sulfate which arecontained within the high-density hematite particles are washed out withwater, even if soluble sodium salt and soluble sulfate adhere to thesurfaces when the surfaces of the hematite particles are coated with acoating material in a subsequent step, for example, the later-describedcoating step, they can be easily removed by water-washing.

[0150] Alternatively, the high-density acicular hematite particlescoated with the oxide of tin or the oxides of tin and antimony may beproduced by the following method. That is, by using acicular goethiteparticles uncoated with the hydroxide of tin or the hydroxides of tinand antimony but coated with the sintering-preventive agent solely as astarting material, high-density acicular hematite particles uncoatedwith the hydroxide of tin or the hydroxides of tin and antimony arefirst produced. The obtained high-density acicular hematite particlesare heated in an aqueous alkaline solution, and then filtered and washedwith water by ordinary methods. Next, the thus treated high-densityacicular hematite particles are coated with the hydroxide of tin or thehydroxides of tin and antimony in the same manner as the above-mentionedcoating-treatment of the acicular goethite particles. Thereafter, thehigh-density acicular hematite particles coated with the hydroxide oftin or the hydroxides of tin and antimony are heated at a temperature ofusually not less than 300° C., preferably 350 to 850° C., to convert thehydroxide of tin or the hydroxides of tin and antimony on surfaces ofthe high-density acicular hematite particles, into the oxide of tin orthe oxides of tin and antimony, thereby obtaining the high-densityacicular hematite particles coated with the oxide of tin or the oxidesof tin and antimony.

[0151] The high-density acicular hematite particles coated with theoxide of tin or the oxides of tin and antimony according to the presentinvention, may be further coated with at least one compound selectedfrom the group consisting of a hydroxide of aluminum, an oxide ofaluminum, a hydroxide of silicon and an oxide of silicon, if required.

[0152] In the coating-treatment, an aluminum compound, a siliconcompound or both the aluminum and silicon compounds are added to a watersuspension obtained by dispersing the high-density acicular hematiteparticles coated with the oxide of tin or the oxides of tin and antimonyin an aqueous solution. The suspension is stirred and if required, thepH value of the suspension is adjusted properly to coat at least a partof the surface of the high-density acicular hematite particles with thehydroxide of aluminum, the oxide of aluminum, the hydroxide of siliconor the oxide of silicon. The suspension is then filtered to separate thecoated high-density acicular hematite particles therefrom. The coatedhigh-density acicular hematite particles is further washed with water,dried and pulverized. If required, the high-density acicular hematiteparticles may be subjected to deaeration, compaction or othertreatments.

[0153] As the aluminum compound for the coating, the same aluminumcompounds as those described above as the sintering preventive areusable.

[0154] The amount of aluminum compound added is usually 0.01 to 50.00 wt% (calculated as Al) based on the weight of the acicular hematiteparticles. When the amount is less than 0.01 wt %, the improvement ofthe dispersibility in the vehicle may be insufficient. On the otherhand, if the amount exceeds 50.00 wt %, the coating effect becomessaturated, so that it is meaningless to add an aluminum compound morethan necessary.

[0155] As the silicon compound, the same silicon compounds as thosedescribed above as the sintering preventive are usable.

[0156] The amount of silicon compound added is usually 0.01 to 50.00 wt% (calculated as SiO₂) based on the weight of the acicular hematiteparticles. When the amount is less than 0.01 wt %, the improvement ofthe dispersibility in the vehicle may be insufficient. On the otherhand, when the amount exceeds 50.00 wt %, the coating effect becomessaturated, so that it is meaningless to add an silicon compound morethan necessary.

[0157] When both an aluminum compound and a silicon compound are used,the amount of thereof used is preferably 0.01 to 50.00 wt % (calculatedas Al and SiO₂) based on the weight of the acicular hematite particles.

[0158] It is important in the present invention that when thehigh-purity and high-density acicular hematite particles in which atleast a part of the surface of the particle is coated with an oxide oftin or oxides of tin and antimony, and, which have an average majoraxial diameter of not more than 0.3 μm, a pH value of not less than 8,and which contain soluble sodium salts of not more than 300 ppm solublesodium (calculated as Na) and soluble sulfates of not more than 150 ppmsoluble sulfate (calculated as SO₄), are used as the non-magneticparticles for a non-magnetic undercoat layer, it is possible to enhancethe strength and the surface smoothness of the non-magnetic undercoatlayer owing to the excellent dispersibility of the high-purity andhigh-density acicular hematite particles into a binder resin; and thatwhen a magnetic recording medium is formed by using the non-magneticundercoat layer, it is possible to reduce the light transmittance andthe surface resistivity, to enhance the strength and to make the surfaceof the magnetic recording layer more smooth. Further, it is possible tosuppress the deterioration in the magnetic properties which is caused bythe corrosion of the acicular magnetic particles containing iron as amain ingredient dispersed in the magnetic recording layer.

[0159] The reason why the strength of the non-magnetic undercoat layeris enhanced and the surface of the non-magnetic undercoat layer is mademore smooth, is considered to be as follows. Since it is possible tosufficiently remove the soluble sodium and the soluble sulfate, whichagglomerate hematite particles by firmly crosslinking, by washing theparticles with water, the agglomerates are separated into substantiallydiscrete particles, so that acicular hematite particles having anexcellent dispersion in the vehicle are obtained.

[0160] This fact will be explained in the following. The goethiteparticles as the starting material are produced by various methods, asdescribed above. When as the raw material for producing aciculargoethite particles ferrous sulfate is used in any method, a large amountof sulfate [SO₄ ²⁻] naturally exists in the goethite suspension.

[0161] Especially, when goethite particles are produced from an acidicsolution, since water-soluble sulfate such as Na₂SO₄ is simultaneouslyproduced and an alkali metal such as K⁺, NH₄ ⁺ and Na⁺ are contained inthe goethite suspension, a deposit containing an alkali metal and asulfate is easily produced. This deposit is represented by RFe₃(SO₄)(OH)₆, (R= K⁺, NH₄ ⁺, or Na⁺). Such a deposit is a slightly solublesulfuric acid-containing salt and cannot be removed by an ordinarywater-washing method. This slightly soluble salt becomes a solublesodium salt or a soluble sulfate in the next heat-treatment step. Thesoluble sodium salt and soluble sulfate are firmly combined with theinteriors or the surfaces of the acicular hematite particles by asintering preventive, which is essential for preventing the deformationof the acicular hematite particles and sintering between particles inthe heat-treatment at a high temperature for the densification of theparticles and which is crosslinking the acicular hematite particles. Inthis manner, agglomeration between acicular hematite particles becomesfurther firmer. As a result, the soluble sulfate and the soluble sodiumsalt, especially, imprisoned in the interiors of the particles or theagglomerates become very difficult to remove by an ordinarywater-washing method.

[0162] When acicular goethite particles are produced in an aqueousalkali solution by using ferrous sulfate and sodium hydroxide, Na₂SO₄ issimultaneously produced as a sulfate and NaOH exists in the goethitesuspension. Since they are both soluble, if the acicular goethiteparticles are adequately washed with water, Na₂SO₄ and NaOH ought to beremoved. However, since the crystallinity of acicular goethite particlesis generally small, the water-washing effect is poor, and when theparticles are washed with water by an ordinary method, the particlesstill contain water-soluble contents such as a soluble sulfate [SO₄ ²⁻]and a soluble sodium salt [Na⁺]. The water-soluble contents are firmlycombined with the interiors or the surfaces of the acicular hematiteparticles by the sintering preventive which is crosslinking theparticles, as described above, and the agglomeration between acicularhematite particles becomes further firmer. As a result, the solublesulfate and the soluble sodium salt, especially, imprisoned in theinteriors of the particles or the agglomerates become very difficult toremove by an ordinary water-washing method.

[0163] It is considered that when the high-density acicular hematiteparticles in which the soluble sodium salt and the soluble sulfate arefirmly combined with the interiors or the surfaces of the particles viathe soluble sintering preventive, as described above, are pulverized bya wet-process so as to deagglomerate coarse particles, and heat-treatedin the aqueous alkali solution having a pH value of not less than 13 ata temperature of not less than 80° C., the aqueous alkali solutionsufficiently permeates into the interiors of the hematite particles, sothat the binding strength of the sintering preventive which is firmlycombined with the interiors and the surfaces of the particles, and theinteriors of the agglomerates is gradually weakened, and thecrosslinking is dissociated from the interiors and the surfaces of theparticles and the interiors of the agglomerates, and simultaneously, thewater-soluble sodium salt and the water-soluble sulfate are easilyremoved by water-washing.

[0164] It is considered that the deterioration in the magneticproperties which is caused by the corrosion of the acicular magneticparticles containing iron as a main ingredient, which are dispersed inthe magnetic recording layer is suppressed because the contents of thesoluble sodium salt and the soluble sulfate, which accelerate thecorrosion of a metal, in the acicular hematite particles are small andthe pH value of the hematite particles themselves is as high as not lessthan 8.

[0165] Actually, it is confirmed that a progress of corrosion ofacicular magnetic particles containing iron as a main ingredient wassuppressed by a synergistic effect of a small soluble content and a pHvalue of not less than 8, from the fact that the advantages of thepresent invention was not attained in any of the cases of (i)heat-treating the hematite particles after wet-pulverization in a slurrywith the pH value adjusted to less than 13 at a temperature of not lessthan 80° C., (ii) heat-treating the hematite particles in a slurry withthe pH value adjusted to not less than 13 at a temperature of less than80° C., or (iii) heat-treating the hematite particles containing coarseparticles without being pulverized by a wet-process in a slurry with thepH value adjusted to not less than 13 at a temperature of not less than80° C., as shown in later-described examples and comparative examples.

[0166] By using the high-density acicular hematite particles accordingto the present invention, a non-magnetic undercoat layer having anexcellent surface smoothness and a uniform thickness because of theirexcellent dispersibility in vehicle, as described above, can beobtained, and a mechanical strength of a substrate when the non-magneticundercoat layer is formed thereon can be improved. Accordingly, thehigh-density acicular hematite particles according to the presentinvention can be suitably used as non-magnetic particles fornon-magnetic undercoat layer.

[0167] Further, by using the non-magnetic undercoat layer according tothe present invention, it becomes possible to form thereon a magneticrecording layer having an excellent surface smoothness and a uniformthickness due to its excellent properties described above. Accordingly,the non-magnetic undercoat layer according to the present invention canbe suitably used as a non-magnetic undercoat layer of a magneticrecording medium for high-density recording.

[0168] Furthermore, the magnetic recording medium according to thepresent invention can exhibit a low transmittance and a low surfaceresistivity, because the high-density acicular hematite particles usedtherein have an excellent dispersibility in vehicle, and are coated withan oxide of tin or oxides of tin and antimony. In addition, since thehigh-density acicular hematite particles have a less soluble sodium saltcontent, a less soluble sulfate content and a pH value of not less than8, acicular magnetic particles containing iron as a main ingredient,dispersed in a magnetic recording layer of the magnetic recordingmedium, can be prevented from being corroded, thereby inhibiting thedeterioration in magnetic properties of the magnetic recording layer.Accordingly, the magnetic recording medium according to the presentinvention can maintain its excellent properties for a long period oftime.

EXAMPLES

[0169] The present invention is described in more detail by Examples andComparative Examples, but the Examples are only illustrative and,therefore, not intended to limit the scope of this invention.

[0170] Various properties of the high-density acicular particles,non-magnetic undercoat layer and magnetic recording medium according tothe present invention were evaluated by the following methods.

[0171] (1) The residue on sieve after the wet-pulverization was obtainedby measuring the concentration of the slurry after pulverization by awet-process in advance, and determining the quantity of the solidcontent on the sieve remaining after the slurry equivalent to 100 g ofthe solid content was passed through the sieve of 325 meshes (mesh size:44 μm).

[0172] (2) The average major axial diameter and the average minor axialdiameter of the particles are expressed by the average values of 350particles measured in the photograph obtained by magnifying an electronmicrograph (×30000) by 4 times in the vertical and horizontaldirections, respectively. The aspect ratio is the ratio of the averagemajor axial diameter and the average minor axial diameter.

[0173] (3) The geometrical standard deviation (σg) of particle sizedistribution of the major axial diameter was obtained by the followingmethod. The major axial diameters of the particles were measured fromthe magnified electron microphotograph in the above-mentioned (2). Theactual major axial diameters of the particles and the number ofparticles were obtained from the calculation on the basis of themeasured values. On logarithmico-normal probability paper, the majoraxial diameters were plotted at regular intervals on the abscissa-axisand the accumulative number of particles belonging to each interval ofthe major axial diameters was plotted by percentage on the ordinate-axisby a statistical technique. The major axial diameters corresponding tothe number of particles of 50% and 84.13%, respectively, were read fromthe graph, and the geometrical standard deviation (σg) was measured fromthe following formula:

Geometrical standard deviation (σg)={major axial diameter (μm)corresponding to 84.13% under integration sieve}/{major axial diameter(geometrical average diameter) corresponding to 50% under integrationsieve}.

[0174] The smaller the geometrical standard deviation, the moreexcellent the particle size distribution of the major axial diameters ofthe particles.

[0175] (4) The specific surface area is expressed by the value measuredby a BET method.

[0176] (5) The decree of denseness of the particles is represented byS_(BET)/S_(TEM) as described above. The S_(BET) is a specific surfacearea measured by the above-described BET method. The S_(TEM) is a valuecalculated from the average major axial diameter d cm and the averageminor axial diameter w cm measured from the electron microphotographdescribed in (2) on the assumption that a particle is a rectangularparallellopiped in accordance with the following formula:

S _(TEM) (m ² /g)={(4·d·w+2w ²)/(d·w ²·ρ_(p))}×10⁻⁴

[0177] wherein ρ_(p) is the true specific gravity of the hematiteparticles, and 5.2 g/cm³ was used.

[0178] Since S_(TEM) is a specific surface area of a particle having asmooth surface without any dehydration pore within or on the surfacethereof, the closer S_(BET)/S_(TEM) of particles is to 1, it means, thesmoother surface the particles have without any dehydration pore withinor in the surface thereof, in other words, the particles arehigh-density particles.

[0179] (6) The content of each of Sn, Sb, Al, Co, P and Si was measuredfrom fluorescent X-ray analysis.

[0180] (7) The pH value of the particles was measured in the followingmethod. 5 g of the sample was weighed into a 300-ml triangle flask, and100 ml of pure water was added. The suspension was heated and afterkeeping the boiled state for 5 minutes, it was corked and left to coolto an ordinary temperature. After adding pure water which was equivalentto the pure water lost by boiling, the flask was corked again, shakenfor 1 minute, and left to stand for 5 minutes. The pH value of thesupernatant obtained was measured in accordance JIS Z 8802-7.

[0181] (8) The contents of soluble sodium salts and soluble sulfateswere measured by measuring the Na content and SO₄ ²⁻ content in thefiltrate obtained by filtering the supernatant liquid produced for themeasurement of pH value which is described above through filter paperNo. 5C, by using an Inductively Coupled Plasma EmissionSpectrophotometer (manufactured by Seiko Instruments and Electronics,Ltd.).

[0182] (9) The volume resistivity of acicular hematite particles wasmeasured as follows. First, 0.5 g of acicular hematite particles wereweighed and pressure-molded under 140 kg/cm² by KBr pellet moldingapparatus (manufactured by Simazu Seisakusho Co., Ltd.) to form acylindrical sample.

[0183] Next, the cylindrical sample was allowed to stand at atemperature of 25° C. and a relative humidity (RH) of 60% for not lessthan 12 hours. The sample was set between stainless steel electrodes,and impressed with a voltage of 15 V using a Wheatstone bridge “TYPE2768” (manufactured by Yokogawa Hokushin Denki Co., Ltd.) to measure aresistance (Ω) thereof.

[0184] Next, the sample was measured for a upper surface area A (cm²)and a thickness t (cm) thereof. A volume resistivity X (Ω·cm) isobtained by the following formula:

X (Ω·cm)=R×(A/t)

[0185] wherein R represents an actual measured value of resistance.

[0186] (10) The surface resistivity of a coating film was measured asfollows. The coating film was first allowed to stand at a temperature of25° C. and a relative humidity (RH) of 60% for not less than 12 hours.Thereafter, the coating film which was slit into a width of 6 mm, andwas disposed between metal electrodes each having a width of 6.5 mm,such that a coating surface of the coating film was contacted with theseelectrodes. 170 g of weights were respectively fixed to opposite ends ofthe coating film so as to bring the coating surface of the coating filminto close contact with the electrodes. Thereafter, D.C. voltage of 500V was applied between the metal electrodes to measure a surfaceresistivity of the coating film by using a resistance meter Model 14329A(manufactured by Yokogawa Hewlett Packard Co., Ltd.).

[0187] (11) The viscosity of the coating composition was obtained bymeasuring the viscosity of the coating composition at 25° C. at a shearrate D of 1.92 sec⁻¹ by using an E type viscometer EMD-R (manufacturedby Tokyo Keiki, Co., Ltd.).

[0188] (12) The gloss of the surface of the coating film of each of thenon-magnetic undercoat layer and the magnetic recording layer wasmeasured at an angle of incidence of 45° by a glossmeter UGV-5D(manufactured by Suga Shikenki, Co., Ltd.).

[0189] (13) The surface roughness Ra is expressed by the average valueof the center-line average roughness of the profile curve of the surfaceof the coating film by using “Surfcom-575A” (manufactured by TokyoSeimitsu Co., Ltd.).

[0190] (14) The strength of the non-magnetic undercoat layer andmagnetic recording medium was expressed the Young's modulus obtained by“Autograph” (produced by Shimazu Seisakusho Ltd.). The Young's moduluswas expressed by the ratio of the Young's modulus of the coating film tothat of a commercially available video tape “AV T-120” (produce byVictor Company of Japan, Ltd.). The higher the relative value, the morefavorable.

[0191] (15) The magnetic properties were measured under an externalmagnetic field of 10 kOe by “Vibration Sample Magnetometer VSM-3S-15”(manufactured by Toei Kogyo, Co., Ltd.).

[0192] (16) The change in the magnetic properties with passage of timeof a magnetic recording medium caused by the corrosion of the magneticparticles containing iron as a main ingredient was examined as follows.

[0193] The magnetic recording medium was allowed to stand in anenvironment of a temperature of 60° C. and a relative humidity of 90%for 14 days, and the coercive force and the saturation magnetic fluxdensity were measured before and after standing. A change in eachcharacteristic was divided by the value before standing, and representedby percentage as a percentage of change.

[0194] (17) The light transmittance of a magnetic recording medium isexpressed by the linear adsorption coefficient using a lighttransmittance at λ=900 nm measured by “Photoelectric SpectrophotometerUV-2100” (manufactured by Shimazu Seisakusho, Ltd.). The linearadsorption coefficient is defined by the following formula:

Linear adsorption coefficient (μm⁻¹)={1n (1/t)}/FT

[0195] wherein t represents light transmittance (−) at λ=900 nm, and FTrepresents thickness (μm) of the coating film composition of the filmused for the measurement.

[0196] The larger the value, the more difficult it is for the magneticrecording medium to transmit light.

[0197] As a blank for measuring the linear adsorption coefficient, thesame non-magnetic substrate as that of the above-mentioned magneticrecording medium, was used.

[0198] (18) The thickness of each of the non-magnetic substrate, thenon-magnetic undercoat layer and the magnetic recording layerconstituting the magnetic recording medium was measured in the followingmanner by using a Digital Electronic Micrometer K351C (manufactured byAnritsu Denki Corp.)

[0199] The thickness (A) of a non-magnetic substrate was first measured.Similarly, the thickness (B) (B=the sum of the thicknesses of thenon-magnetic substrate and the non-magnetic undercoat layer) of asubstrate obtained by forming a non-magnetic undercoat layer on thenon-magnetic substrate was measured. Furthermore, the thickness (C)(C=the sum of the thicknesses of the non-magnetic substrate, thenon-magnetic undercoat layer and the magnetic recording layer) of amagnetic recording medium obtained by forming a magnetic recording layeron the non-magnetic undercoat layer was measured. The thickness of thenon-magnetic undercoat layer is expressed by B−A, and the thickness ofthe magnetic recording layer is expressed by C−B.

Example 1

[0200] <production of acicular hematite particles>

Example 1

[0201] 1,200 g of spindle-shaped goethite particles obtained by theafore-mentioned goethite production method (B) (average major axialdiameter: 0.178 μm, average minor axial diameter: 0.0225 μm, aspectratio: 7.91:1, BET specific surface area: 160.3 m²/g, soluble sodiumsalt content: 1232 ppm (calculated as Na), soluble sulfate content: 621ppm (calculated as SO₄), pH value: 6.7 and geometrical standarddeviation: 1.33) were suspended in a mixed solution of an aqueousferrous sulfate solution and an aqueous sodium carbonate solution toform a slurry having a solid content of 8 g/liter. After 150 liters ofthe slurry was heated to 60° C., a 0.1N NaOH aqueous solution was addedthereto to adjust the pH value to 9.0.

[0202] Next, 2,022 ml of an aqueous solution containing 0.5 mol/liter ofsodium stannate was gradually added to the thus obtained alkalineslurry. After completion of the addition, a 0.8N acetic acid solutionwas added to the slurry to adjust the pH value to 7.5. Thereafter, theslurry was successively filtered, washed with water, dried andpulverized by ordinary methods to obtain spindle-shaped goethiteparticles whose surfaces were coated with a hydroxide of tin. It wasconfirmed that the amount of the hydroxide of tin was 9.32% by weight(calculated as Sn) based on the weight of the spindle-shaped goethiteparticles.

[0203] 1,000 g of the thus obtained spindle-shaped goethite particlescoated with the hydroxide of tin, were charged into a stainless steelrotary furnace, and heat-dehydrated in air at 350° C. for 60 minuteswhile rotating the furnace, thereby obtaining low-density spindle-shapedhematite particles coated with an oxide of tin. It was determined thatthe thus obtained low-density spindle-shaped hematite particles had anaverage major axial diameter of 0.134 μm, an average minor axialdiameter of 0.0194 μm, an aspect ratio (average major axialdiameter/average minor axial diameter) of 6.91:1, a BET specific surfacearea (S_(BET)) of 168.3 m²/g, a degree of densification(S_(BET)/S_(TEM)) of 3.96, a soluble sodium salt content of 1123 ppm(calculated as Na), a soluble sulfate content of 465 ppm (calculated asSO₄), a pH value of 6.3, a geometrical standard deviation of 1.34 and avolume resistivity of 1.1×10⁶ Ωcm. Further, the amount of the oxide oftin was 10.60% by weight (calculated as Sn) based on the weight of thespindle-shaped hematite particles.

[0204] Next, 900 g of the low-density spindle-shaped hematite particleswere charged into a ceramic rotary furnace, and heated in air at 650° C.for 20 minutes while rotating the furnace to seal dehydrating pores ofthe particles, thereby obtaining high-density spindle-shaped hematiteparticles coated with the oxide of tin. It was determined that theobtained high-density spindle-shaped hematite particles had an averagemajor axial diameter of 0.129 μm, an average minor axial diameter of0.0206 μm, an aspect ratio (average major axial diameter/average minoraxial diameter) of 6.26:1, a BET specific surface area (S_(BET)) of 46.6m²/g, a degree of high densification (S_(BET)/S_(TEM)) of 1.16, asoluble sodium salt content of 2864 ppm (calculated as Na), a solublesulfate content of 2956 ppm (calculated as SO₄), a pH value of 5.4, ageometrical standard deviation of 1.36 and a volume resistivity of9.6×10⁵ Ωcm. Further, the amount of the oxide of tin was 10.72% byweight (calculated as Sn) based on the weight of the spindle-shapedhematite particles.

[0205] After 800 g of the high-density spindle-shaped hematite particlesobtained were roughly pulverized by a Nara mill in advance, the obtainedparticles were charged into 4.7 liters of pure water and peptized by ahomomixer (manufactured by Tokushu-kika Kogyo, CO., Ltd.) for 60minutes.

[0206] The slurry of the high-density spindle-shaped hematite particlesobtained was then mixed and dispersed for 3 hours at an axial rotationfrequency of 2000 rpm while being circulated by a horizontal SGM(Dispermat SL, manufactured by S. C. Adichem, CO., Ltd.). Thespindle-shaped hematite particles in the slurry remaining on a sieve of325 meshes (mesh size: 44 μm) was 0% by weight.

[0207] The concentration of the high-density spindle-shaped hematiteparticles in the slurry was adjusted to 100 g/liter, and a 6N-aqueousNaOH solution was added to 7 liter of the slurry under stirring so as toadjust the pH value to 13.3. The slurry was then heated to 95° C. understirring, and was held for 3 hours at 95° C.

[0208] The slurry was then washed with water by a decantation method andthe pH value of the slurry was adjusted to 10.5. When the concentrationof the slurry at this point was checked so as to ensure the accuracy, itwas 96 g/liter.

[0209] 2 liter of the slurry washed with water was filtered through aBuchner filter, and pure water was passed until the electricconductivity of the filtrate became not more than 30 μs. The particleswere then dried by an ordinary method and pulverized so as to obtain thetarget high-density spindle-shaped hematite particles. The high-densityspindle-shaped hematite particles obtained contained 10.83 wt % of anoxide of tin (calculated as Sn), and had an average major axial diameterof 0.128 μm, a minor axial diameter of 0.0206 μm, an aspect ratio of6.71:1, a geometric standard deviation σg of particle size (major axialdiameter) of 1.35, a BET specific surface (S_(BET)) of 47.1 m²/g, aS_(BET)/S_(TEM) value of densification of 1.17 and a pH value of theparticles of 8.9. The spindle-shaped hematite particles containedsoluble sodium salts of 112 ppm soluble sodium (calculated as Na) andsoluble sulfates of 41 ppm soluble sulfate (calculated as SO₄). Thevolume resistivity thereof was 6.3×10⁶ Ωcm.

Example 2

[0210] <Production of a non-magnetic undercoat layer>

[0211] 12 g of the high-density spindle-shaped hematite particlesobtained in the Example 1 were mixed with a binder resin solution (30 wt% of vinyl chloride-vinyl acetate copolymer resin having a sodiumsulfonate group and 70 wt % of cyclohexanone) and cyclohexanone, and themixture (solid content: 72 wt %) obtained was kneaded by a plasto-millfor 30 minutes.

[0212] The thus-obtained kneaded material was charged into a 140ml-glass bottle together with 95 g of 1.5 mmφ glass beads, a binderresin solution (30 wt % of polyurethane resin having a sodium sulfonategroup and 70 wt % of a solvent (methyl ethyl ketone:toluene=1:1)),cyclohexanone, methyl ethyl ketone and toluene, and the obtained mixturewas mixed and dispersed by a paint shaker for 6 hours to obtain acoating composition.

[0213] The thus-obtained coating composition containing high-densityspindle-shaped hematite particles was as follows: High-densityspindle-shaped 100 parts by weight hematite particles Vinylchloride-vinyl acetate 10 parts by weight copolymer resin having asodium sulfonate group Polyurethane resin having a 10 parts by weightsodium sulfonate group Cyclohexanone 44.6 parts by weight Methylethylketone 111.4 parts by weight Toluene 66.9 parts by weight

[0214] The coating composition obtained containing high-densityspindle-shaped hematite particles was applied to a polyethyleneterephthalate film of 12 μm thick to a thickness of 55 μm by anapplicator, and the film was then dried, thereby forming a non-magneticundercoat layer. The thickness of the non-magnetic undercoat layer was3.5 μm.

[0215] The gloss of the coating film of the obtained non-magneticundercoat layer was 201%, the surface roughness Ra was 6.8 nm, and theYoung's modulus (relative value) was 128.

Example 3

[0216] <Production of a magnetic recording layer>

[0217] 12 g of spindle-shaped magnetic particles containing iron as amain ingredient (average major axial diameter: 0.104 μm, average minoraxial diameter: 0.0158 μm, aspect ratio: 6.58:1, coercive force: 1905Oe, saturation magnetization: 138 emu/g, Al content: 4.41 wt %, and Cocontent: 5.51 wt %), 1.2 g of a polishing agent (AKP-50: trade name,produced by Sumitomo Chemical Co., Ltd.), 0.36 g of carbon black(#3250B, trade name, produced by Mitsubishi Chemical Corp.), a binderresin solution (30 wt % of vinyl chloride-vinyl acetate copolymer resinhaving a sodium sulfonate group and 70 wt % of cyclohexanone) andcyclohexanone were mixed to obtain a mixture (solid content: 78 wt %).The mixture was further kneaded by a plasto-mill for 30 minutes toobtain a kneaded material.

[0218] The thus-obtained kneaded material was charged into a 140ml-glass bottle together with 95 g of 1.5 mmφ glass beads, a binderresin solution (30 wt % of polyurethane resin having a sodium sulfonategroup and 70 wt % of a solvent (methyl ethyl ketone:toluene=1:1)),cyclohexanone, methyl ethyl ketone and toluene, and the mixture wasmixed and dispersed by a paint shaker for 6 hours. Thereafter, thelubricant and hardening agent were added to the resultant mixture, andthen the obtained mixture was mixed and dispersed by paint shaker for 15minutes.

[0219] The thus-obtained magnetic coating composition was as follows:Magnetic particles containing 100 parts by weight iron as a mainingredient Vinyl chloride-vinyl acetate 10 parts by weight copolymerresin having a sodium sulfonate group Polyurethane resin having a 10parts by weight sodium sulfonate group Polishing agent (AKP-50) 10 partsby weight Carbon black (#3250B) 3.0 parts by weight Lubricant (myristicacid: butyl 3.0 parts by weight stearate 1:2) Hardening agent(polyisocyanate) 5.0 parts by weight Cyclohexanone 65.8 parts by weightMethyl ethyl ketone 164.5 parts by weight Toluene 98.7 parts by weight

[0220] The magnetic coating composition obtained was applied to thenon-magnetic undercoat layer to a thickness of 15 μm by an applicator,and the magnetic recording medium obtained was oriented and dried in amagnetic field, and then calendered. The magnetic recording medium wasthen subjected to a curing reaction at 60° C. for 24 hours, andthereafter slit into a width of 0.5 inch, thereby obtaining a magnetictape. The thickness of the magnetic recording layer was 1.1 μm.

[0221] The magnetic tape obtained had a coercive force of 1986 Oe, asquareness (Br/Bm) of 0.87, a gloss of 228%, a surface roughness Ra of6.4 nm, a Young's modulus (relative value) of 133, a linear absorptioncoefficient of 1.21, a surface resistivity of 1.1×10⁷ Ω/sq. The changesin the coercive force and the saturation magnetic flux density Bm withpassage of time were 6.4%, and 5.4%, respectively.

Examples 4 to 21, Comparative Examples 1 to 14

[0222] <Types of acicular goethite particles>

[0223] The starting materials A to F shown in Table 1 were used as thestarting materials for producing acicular hematite particles.

[0224] <Production of low-density acicular hematite articles>

[0225] Low-density acicular hematite particles were obtained in the sameway as in Example 1 except for varying the kind of acicular goethiteparticles as the starting materials, the kind and amount of tincompound, the kind and amount of antimony compound, the kind and amountof sintering preventive, and heat-dehydrating temperature and time.

[0226] The main producing conditions and various properties are shown inTables 2 to 3.

Examples 22 to 39. Comparative Examples 15 to 27

[0227] <Production of high-density acicular hematite particles>

[0228] High-density acicular hematite particles were obtained in thesame way as in Example 1 except for varying the kind of low-densityhematite particles, and the heating temperature and time fordensification.

[0229] The main producing conditions and various properties are shown inTables 4 and 5.

Examples 40 to 57, Comparative Examples 28 to 35

[0230] <Treatment of acicular hematite particles in an aqueous alkalisolution>

[0231] High-purity, high-density acicular hematite particles wereobtained in the same way as in Example 1 except for varying the kind ofhigh-density acicular hematite particles, whether or not thewet-pulverization process was conduced, whether or not theheat-treatment in the aqueous alkali solution was conducted, the pHvalue of the slurry, and the heating time and temperature.

[0232] The main producing conditions and various properties are shown inTables 6 to 9.

Example 58

[0233] <Surface coating of acicular hematite particles>

[0234] The concentration of the slurry having a pH value 10.5 which wasobtained in Example 40 by washing the particles in an aqueous alkalisolution after heat-treatment with water by a decantation method was 96g/liter. 5 liter of the slurry was re-heated to 60° C., and 231 ml(equivalent to 1.3 wt % (calculated as Al) based on the acicularhematite particles) of a 1.0-N NaAlO₂ solution was added to the slurry,and the mixture was held for 60 minutes. Thereafter, the pH value of themixture was adjusted to 8.2 by using acetic acid. The particles werethen filtered out, washed with water, dried and pulverized in the sameway as in Example 1, thereby obtaining acicular hematite particlescoated with a coating material.

[0235] The main producing conditions and various properties are shown inTables 10 and 11.

Examples 59 to 72

[0236] Acicular hematite particles coated with a coating material wereobtained in the same way as in Example 58 except for varying the kind ofacicular hematite particles, and the kind and the amount of surfacetreating material.

[0237] The main producing conditions and various properties are shown inTable 10 and 11.

Example 73

[0238] <Coating-treatment of high-density acicular hematite particlestreated with alkaline aqueous solution, with oxide of tin or oxides oftin and antimony>

[0239] The slurry obtained in Example 55 by washing with water by adecantation method after the heat-treatment in alkaline aqueoussolution, had a pH value of 10.5 and a concentration of 96 g/liter.After 5 liters of the slurry was heated again to 60° C., 121 ml of a1.0-mol sodium stannate solution (corresponding to 3.0% by weight(calculated as Sn) based on the weight of the acicular hematiteparticles) was added thereto. After the slurry was allowed to stand for60 minutes, the pH value thereof was adjusted to 8.0 by adding aceticacid thereto. Next, the slurry was filtered to separate a solidcomponent therefrom, and then the solid component was washed with water,dried and pulverized in the same manner as in Example 1, therebyobtaining high-density acicular hematite particles coated with ahydroxide of tin.

[0240] The essential production conditions and properties of theobtained high-density acicular hematite particles are shown in Tables 10and 11.

Examples 74 and 75

[0241] The same procedure as defined in Example 73 was conducted exceptthat kind of acicular hematite particles treated with alkaline aqueoussolution, kind and amount of tin compound, kind, amount and use ornon-use of antimony compound and treating temperature and treating timeused for the heat-treatment were varied, thereby obtaining high-densityacicular hematite particles coated with a hydroxide of tin or hydroxidesof tin and antimony.

[0242] The essential production conditions and properties of theobtained high-density acicular hematite particles are shown in Tables 10and 11.

Example 76

[0243] The high-density acicular hematite particles coated with thehydroxide of tin which were obtained in Example 73, were charged into astainless steel rotary furnace, and heated in air at 400° C. for 60minutes while rotating the furnace, thereby obtaining high-densityacicular hematite particles coated with an oxide of tin.

[0244] The essential production conditions and properties of theobtained high-density acicular hematite particles are shown in Tables 12and 13.

Examples 77 and 78

[0245] The same procedure as defined in Example 76 was conducted exceptthat kind of high-density acicular hematite particles coated with ahydroxide of tin or hydroxides of tin and antimony and treatingtemperature and treating time used for the heat-treatment were varied,thereby obtaining high-density acicular hematite particles coated withan oxide of tin or oxides of tin and antimony.

[0246] The essential production conditions and properties of theobtained high-density acicular hematite particles are shown in Tables 12and 13.

Examples 79 to 111, Comparative Examples 36 to 50

[0247] <Production of a non-magnetic undercoat layer>

[0248] A non-magnetic undercoat layer was obtained in the same way as inExample 2 by using the acicular hematite particles obtained in Examples40 to 54, 58 to 72 and 76 to 78, Comparative Examples 1, 3, 15 to 18, 23and 28 to 35.

[0249] The main producing conditions and various properties are shown inTables 14 to 16.

Examples 112 to 144, Comparative Examples 51 to 65

[0250] <Production of a magnetic recording medium using magneticparticles containing iron as a main ingredient>

[0251] A magnetic recording medium using acicular magnetic particlescontaining iron as a main ingredient was obtained in the same way as inExample 3 except for varying the kind of non-magnetic undercoat layerobtained in Examples 79 to 111 and Comparative Examples 36 to 50 and thekind of acicular magnetic particles containing iron as a mainingredient.

[0252] The main producing conditions and various properties are shown inTables 17 to 19. TABLE 1 Acicular Goethite particles Kind of Averagemajor Average minor starting Production axial axial material methoddiameter (μm) diameter (μm) Starting BB 0.181 0.0246 material A StartingBB 0.220 0.0283 material B Starting DD 0.245 0.0305 material C StartingCC 0.164 0.0218 material D Starting AA 0.260 0.0298 material E StartingBB 0.234 0.0288 material F Acicular Goethite particles Geometrical Kindof standard BET specific starting Aspect ratio* deviation σg surfacearea material (−) (−) (m²/g) Starting 7.36 1.37 151.0 material AStarting 7.77 1.34 125.0 material B Starting 8.03 1.31 95.1 material CStarting 7.52 1.37 186.5 material D Starting 8.72 1.44 72.6 material EStarting 8.13 1.31 110.5 material F Acicular Goethite particles Kind ofSoluble starting sodium salt Soluble pH value material (ppm) sulfate(ppm) (−) Starting 412 386 6.8 material A Starting 512 264 7.2 materialB Starting 1215 2150 5.1 material C Starting 415 915 5.5 material DStarting 1565 171 8.3 material E Starting 436 312 6.9 material F

[0253] (Note) PRODUCTION METHOD:

[0254] AA: A method of oxidizing a suspension having a pH value of notless than 11 and containing colloidal ferrous hydroxide particles whichis obtained by adding not less than an equivalent of an alkali hydroxidesolution to an aqueous ferrous salt solution, by passing anoxygen-containing gas thereinto at a temperature of not higher than 80°C.

[0255] BB: A method of producing acicular goethite particles byoxidizing a suspension containing FeCO₃ which is obtained by reacting anaqueous ferrous salt solution with an aqueous alkali carbonate solution,by passing an oxygen-containing gas thereinto after aging thesuspension, if necessary.

[0256] CC: A method of growing acicular seed goethite particles byoxidizing a ferrous hydroxide solution containing colloidal ferroushydroxide particles which is obtained by adding less than an equivalentof an alkali hydroxide solution or an alkali carbonate solution to anaqueous ferrous salt solution, by passing an oxygen-containing gasthereinto, thereby producing acicular seed goethite particles, addingnot less than an equivalent of an alkali hydroxide solution to the Fe²⁺in the aqueous ferrous salt solution, to the aqueous ferrous saltsolution containing the acicular goethite seed particles, and passing anoxygen-containing gas into the aqueous ferrous salt solution.

[0257] DD: A method of growing acicular seed goethite particles byoxidizing a ferrous hydroxide solution containing colloidal ferroushydroxide particles which is obtained by adding less than an equivalentof an alkali hydroxide solution or an alkali carbonate solution to anaqueous ferrous salt solution, by passing an oxygen-containing gasthereinto, thereby producing acicular seed goethite particles, andgrowing the obtained acicular seed goethite particles in an acidic orneutral region. TABLE 2 Kind of acicular Sintering preventive goethiteparticles Calcu- Amount as the starting lated added Examples materialKind as (wt. %) Example 4 Particles of Sodium stannate Sn 15.0 Example 1Example 5 Starting material A Stannous chloride Sn 20.0 Example 6Starting material A Sodium stannate Sn 50.0 #3 Water glass SiO₂ 1.0Example 7 Starting material B Sodium stannate Sn 1.2 Example 8 Startingmaterial B Sodium stannate Sn 3.0 Example 9 Starting material C Stannouschloride Sn 10.0 Example 10 Starting material C Sodium stannate Sn 50.0Example 11 Starting material D Sodium stannate Sn 200.0 Example 12Starting material D Sodium stannate Sn 500.0 Example 13 Startingmaterial E Stannous chloride Sn 100.0 Example 14 Starting material ESodium stannate Sn 75.0 Phosphoric acid P 0.5 Example 15 Startingmaterial F Sodium stannate Sn 50.0 Antimony Sb 5.0 chloride Example 16Starting material F Sodium stannate Sn 10.0 Antimony acetate Sb 2.0Example 17 Particles of Sodium stannate Sn 100.0 Example 1 Antimony solSb 10.0 #3 Water glass SiO₂ 1.5 Example 18 Particles of Stannouschloride Sn 300.0 Example 1 Antimony Sb 15.0 chloride Aluminum sulfateAl 1.0 Example 19 Starting material A Sodium P 1.5 hexameta- phosphateExample 20 Starting material A #3 Water glass SiO₂ 2.0 Example 21Starting material A Phosphoric acid P 0.5 #3 Water glass SiO₂ 0.75Low-density acicular hematite Heat treatment for low particlesdensification Average major Average minor Temperature Time axialdiameter axial diameter Examples (° C.) (min) (μm) (μm) Example 4 300 900.138 0.0199 Example 5 350 30 0.143 0.0214 Example 6 330 120  0.1400.0221 Example 7 280 30 0.180 0.0261 Example 8 300 60 0.183 0.0260Example 9 330 120  0.200 0.0277 Example 10 350 90 0.206 0.0290 Example11 380 60 0.138 0.0211 Example 12 400 30 0.140 0.0240 Example 13 380 600.218 0.0283 Example 14 380 90 0.213 0.0276 Example 15 350 60 0.1910.0277 Example 16 375 60 0.192 0.0278 Example 17 310 240  0.142 0.0205Example 18 350 180  0.145 0.0211 Example 19 330 60 0.141 0.0213 Example20 310 90 0.141 0.0210 Example 21 350 120  0.144 0.0278 Low-densityacicular hematite particles Geometrical standard Aspect ratio* S_(BET)S_(TEM) Examples deviation σg (−) (−) (m²/g) (m²/g) Example 4 1.33 6.93169.1 41.4 Example 5 1.37 6.68 168.5 38.6 Example 6 1.38 6.33 160.8 37.6Example 7 1.34 6.90 144.0 31.6 Example 8 1.34 7.04 135.9 31.7 Example 91.33 7.22 119.0 29.7 Example 10 1.32 7.10 125.1 28.4 Example 11 1.376.54 197.5 39.2 Example 12 1.40 5.83 213.5 34.8 Example 13 1.44 7.70110.8 28.9 Example 14 1.44 7.72 103.6 29.7 Example 15 1.32 6.90 146.929.8 Example 16 1.32 6.91 138.5 29.7 Example 17 1.34 6.93 173.8 40.2Example 18 1.34 6.87 180.6 39.1 Example 19 1.36 6.62 171.6 38.8 Example20 1.36 6.71 178.8 39.4 Example 21 1.36 6.92 186.2 39.7 Low-densityacicular hematite particles S_(BET)/S_(TEM) Soluble sodium Solublesulfate pH value Examples (−) salt (ppm) (ppm) (−) Example 4 4.08 1263680 7.8 Example 5 4.36 1897 879 7.1 Example 6 4.28 1835 980 8.1 Example7 4.56  870 789 6.5 Example 8 4.29 1123 987 6.8 Example 9 4.01 23561234  6.3 Example 10 4.41 2987 1145  6.8 Example 11 5.03 3456 789 7.9Example 12 6.14 4890 891 8.0 Example 13 3.83 2156 1348  6.5 Example 143.49 1768 1123  7.5 Example 15 4.93 1345 888 6.0 Example 16 4.67 18791334  6.1 Example 17 4.32 2350 1345  8.1 Example 18 4.62 2879 789 7.3Example 19 4.42 2006 912 7.5 Example 20 4.54 2128 986 7.3 Example 214.70 2166 982 7.3

[0258] TABLE 3 Kind of Sintering preventive acicular goethite Calcu-Amount Comparative particles as the lated added Examples startingparticles Kind as (wt. %) Comparative Particles of — — — Example 1Example 1 Comparative Particles of — — — Example 2 Example 1 ComparativeParticles of #3 Water glass SiO₂ 0.50 Example 3 Example 1 ComparativeParticles of Phosphoric acid P 0.50 Example 4 Example 1 ComparativeParticles of #3 Water glass SiO₂ 1.00 Example 5 Example 1 ComparativeParticles of Sodium P 0.50 Example 6 Example 1 hexameta- phosphateComparative Particles of #3 Water glass SiO₂ 1.50 Example 7 Example 1Comparative Particles of #3 Water glass SiO₂ 0.20 Example 8 Example 1Comparative Particles of Phosphoric acid P 0.75 Example 9 Example 1Comparative Starting material F Sodium P 2.00 Example 10 hexameta-phosphate Comparative Starting material F #3 Water glass SiO₂ 1.25Example 11 Comparative Starting material F Phosphoric acid P 1.50Example 12 Comparative Starting material F Colloidal silica SiO₂ 0.25Example 13 Comparative Starting material F Sodium stannate Sn 0.05Example 14 Low-density acicular hematite Heat treatment for lowparticles densification Average major Average minor ComparativeTemperature Time axial diameter axial diameter Examples (° C.) (min)(μm) (μm) Comparative 350 90 0.135 0.0199 Example 1 Comparative 380 450.132 0.0206 Example 2 Comparative 340 75 0.136 0.0197 Example 3Comparative — — — — Example 4 Comparative 350 60 0.134 0.0199 Example 5Comparative 350 30 0.132 0.0197 Example 6 Comparative 330 90 0.1340.0197 Example 7 Comparative 300 60 0.132 0.0190 Example 8 Comparative380 20 0.132 0.0195 Example 9 Comparative 380 90 0.193 0.0276 Example 10Comparative 350 90 0.193 0.0280 Example 11 Comparative 330 30 0.1920.0278 Example 12 Comparative 325 45 0.189 0.0288 Example 13 Comparative300 60 0.185 0.0293 Example 14 Low-density acicular hematite particlesGeometrical Comparative standard Aspect ratio* S_(BET) S_(TEM) Examplesdeviation σg (−) (−) (m²/g) (m²/g) Comparative 1.33 6.78 171.6 41.5Example 1 Comparative 1.34 6.41 135.8 40.3 Example 2 Comparative 1.336.90 165.8 41.9 Example 3 Comparative — — — — Example 4 Comparative 1.346.73 134.8 41.5 Example 5 Comparative 1.35 6.70 125.9 42.0 Example 6Comparative 1.35 6.80 145.0 41.9 Example 7 Comparative 1.35 6.95 145.943.4 Example 8 Comparative 1.36 6.77 156.9 42.4 Example 9 Comparative1.33 6.99 124.3 29.9 Example 10 Comparative 1.33 6.89 131.2 29.5 Example11 Comparative 1.32 6.91 138.2 29.7 Example 12 Comparative 1.33 6.56136.3 28.7 Example 13 Comparative 1.32 6.31 126.4 28.3 Example 14Low-density acicular hematite particles Soluble Soluble VolumeComparative S_(BET)/S_(TEM) sodium salt sulfate pH value resistivityExamples (−) (ppm) (ppm) (−) (Ωcm) Comparative 4.13  852 568 6.0 1.2 ×10⁸ Example 1 Comparative 3.37  903 498 6.5 — Example 2 Comparative 3.961245 423 6.8 4.0 × 10⁸ Example 3 Comparative — — — — — Example 4Comparative 3.25 1235 568 6.7 — Example 5 Comparative 3.00 1025 612 6.6— Example 6 Comparative 3.46 1365 682 7.1 — Example 7 Comparative 3.361265 591 7.2 — Example 8 Comparative 3.70 1124 654 7.1 — Example 9Comparative 4.16 1026 689 6.9 — Example 10 Comparative 4.45 1176 563 7.1— Example 11 Comparative 4.66 1015 597 7.0 — Example 12 Comparative 4.74 892 498 6.2 — Example 13 Comparative 4.46 1235 569 7.3 — Example 14

[0259] TABLE 4 High-density acicular hematite particles Kind of low-Heat treatment for Average Average density acicular high densificationmajor axial minor axial hematite Tempera- Time diameter diameterExamples particles ture (° C.) (min) (μm) (μm) Example 22 Example 4 70060 0.132 0.0210 Example 23 Example 5 730 60 0.138 0.0223 Example 24Example 6 750 60 0.138 0.0230 Example 25 Example 7 600 15 0.169 0.0301Example 26 Example 8 610 15 0.173 0.0288 Example 27 Example 9 650 300.186 0.0294 Example 28 Example 10 730 90 0.204 0.0315 Example 29Example 11 750 120  0.137 0.0222 Example 30 Example 12 800 30 0.1390.0243 Example 31 Example 13 750 30 0.211 0.0299 Example 32 Example 14750 60 0.208 0.0296 Example 33 Example 15 730 60 0.189 0.0285 Example 34Example 16 710 45 0.189 0.0298 Example 35 Example 17 750 30 0.139 0.0220Example 36 Example 18 780 60 0.140 0.0219 Example 37 Example 19 680 600.141 0.0213 Example 38 Example 20 700 30 0.140 0.0211 Example 39Example 21 700 60 0.142 0.0210 High-density acicular hematite particlesGeometrical standard Aspect S_(BET) S_(TEM) S_(BET)/S_(TEM) Examplesdeviation σg (−) ratio* (−) (m²/g) (m²/g) (−) Example 22 1.35 6.29 51.339.5 1.30 Example 23 1.38 6.19 46.8 37.3 1.26 Example 24 1.38 6.00 44.936.2 1.24 Example 25 1.35 5.61 38.5 27.8 1.38 Example 26 1.35 6.01 35.128.9 1.21 Example 27 1.35 6.33 33.7 28.2 1.19 Example 28 1.34 6.48 34.926.3 1.33 Example 29 1.39 6.17 51.0 37.5 1.36 Example 30 1.41 5.72 53.234.4 1.55 Example 31 1.44 7.06 37.5 27.5 1.36 Example 32 1.45 7.03 38.927.8 1.40 Example 33 1.33 6.63 40.1 29.0 1.38 Example 34 1.34 6.34 37.527.8 1.35 Example 35 1.36 6.32 55.9 37.7 1.48 Example 36 1.35 6.39 57.137.9 1.51 Example 37 1.36 6.62 51.2 38.8 1.32 Example 38 1.36 6.64 50.639.2 1.29 Example 39 1.36 6.76 51.6 39.3 1.31 High-density acicularhematite particles Kind of Sintering preventive Calculated AmountCalculated Amount Calculated Amount Examples as (wt. %) as (wt. %) as(wt. %) Example 22 Sn 14.33 — — — — Example 23 Sn 18.06 — — — — Example24 Sn 36.83 — — SiO₂ 0.62 Example 25 Sn  1.30 — — — — Example 26 Sn 3.16 — — — — Example 27 Sn 10.06 — — — — Example 28 Sn 35.81 — — — —Example 29 Sn 73.31 — — — — Example 30 Sn 91.68 — — — — Example 31 Sn55.68 — — — — Example 32 Sn 47.15 P 0.48 — — Example 33 Sn 37.13 Sb 5.14— — Example 34 Sn  9.65 Sb 2.01 — — Example 35 Sn 49.30 Sb 8.88 SiO₂1.41 Example 36 Sn 80.61 Sb 14.31  Al 0.96 Example 37 — — P 1.36 — —Example 38 — — — — SiO₂ 1.83 Example 39 — — P 0.51 SiO₂ 0.70High-density acicular hematite particles Soluble sodium salt Solublesulfate pH value Examples (ppm) (ppm) (−) Example 22 1894 3400 5.1Example 23 2561 3604 4.6 Example 24 2569 3448 5.7 Example 25 1205 36985.6 Example 26 1640 3948 5.2 Example 27 3063 3702 4.7 Example 28 37643591 5.8 Example 29 4182 3077 6.5 Example 30 5477 3475 6.8 Example 312911 4044 4.1 Example 32 2581 3931 4.5 Example 33 2219 2664 5.0 Example34 2743 4669 4.1 Example 35 3485 2356 7.8 Example 36 3023 2768 6.8Example 37 2682 3162 5.5 Example 38 2766 3082 5.8 Example 39 2826 33655.5

[0260] TABLE 5 Kind of low- High-density acicular density acicular Heattreatment for hematite particles hematite high densification AverageAverage particles or Tempera- major axial minor axial Comparativeacicular goethite ture Time diameter diameter Examples particles (° C.)(min) (μm) (μm) Comparative Example 1 720 15 0.075 0.0336 Example 15Comparative Comparative 680 15 0.098 0.0286 Example 16 Example 2Comparative Comparative 700 30 0.125 0.0248 Example 17 Example 4Comparative Comparative 750 60 0.131 0.0220 Example 18 Example 5Comparative Comparative 570 90 0.134 0.0206 Example 19 Example 6Comparative Comparative 720 45 0.134 0.0201 Example 20 Example 7Comparative Comparative 730 30 0.132 0.0218 Example 21 Example 8Comparative Comparative 520 90 0.132 0.0198 Example 22 Example 9Comparative Comparative 720 15 0.191 0.0294 Example 23 Example 10Comparative Comparative 650 40 0.192 0.0288 Example 24 Example 11Comparative Comparative 600 15 0.192 0.0290 Example 25 Example 12Comparative Comparative 750 20 0.190 0.0300 Example 26 Example 13Comparative Comparative 460 60 0.185 0.0292 Example 27 Example 14High-density acicular hematite particles Geometrical Aspect Comparativestandard ratio* S_(BET) S_(TEM) S_(BET)/S_(TEM) Examples deviation σg(−) (−) (m²/g) (m²/g) (−) Comparative 1.84 2.23 15.8 28.0 0.56 Example15 Comparative 1.71 3.43 21.9 30.8 0.71 Example 16 Comparative 1.56 5.0431.8 34.1 0.93 Example 17 Comparative 1.36 5.95 45.6 37.9 1.20 Example18 Comparative 1.35 6.50 59.3 40.2 1.47 Example 19 Comparative 1.35 6.6752.6 41.1 1.28 Example 20 Comparative 1.36 6.06 43.6 38.2 1.14 Example21 Comparative 1.34 6.67 68.9 41.8 1.65 Example 22 Comparative 1.34 6.5035.2 28.2 1.25 Example 23 Comparative 1.33 6.67 43.9 28.7 1.53 Example24 Comparative 1.33 6.62 51.5 28.5 1.81 Example 25 Comparative 1.34 6.3337.5 27.7 1.36 Example 26 Comparative 1.33 6.34 65.0 28.4 2.29 Example27 High-density acicular hematite particles Kind of Sintering preventiveSoluble Soluble Volume Comparative Calculated Amount sodium sulfate pHvalue resistivity Example as (wt. %) salt (ppm) (ppm) (−) (Ωcm)Comparative — — 1658 3256 5.3 5.6 × 10⁸ Example 15 Comparative — — 17453569 5.3 8.9 × 10⁸ Example 16 Comparative P 0.44 1652 3756 5.1 3.8 × 10⁸Example 17 Comparative SiO₂ 0.93 1548 3874 4.9 7.1 × 10⁸ Example 18Comparative P 0.46 1436 2964 5.3 — Example 19 Comparative SiO₂ 1.40 15693684 5.1 — Example 20 Comparative SiO₂ 0.21 1856 3548 5.2 — Example 21Comparative P 0.74 1329 2456 5.6 — Example 22 Comparative P 1.83 19543659 5.0 3.8 × 10⁸ Example 23 Comparative SiO₂ 1.22 2045 3246 5.6 —Example 24 Comparative P 1.46 1564 2857 5.2 — Example 25 ComparativeSiO₂ 0.23 1186 3156 4.7 — Example 26 Comparative Sn 0.05 2356 3247 5.6 —Example 27

[0261] TABLE 6 Kind of high- Wet-pulverization Heat treatment densitystep in aqueous acicular Residue alkaline solution hematite Use or onsieve pH Tempera- Time Examples particles non-use (wt. %) (−) ture (°C.) (min) Example 40 Example 22 used 0 13.1 98 180 Example 41 Example 23used 0 13.5 94 180 Example 42 Example 24 used 0 13.3 95 180 Example 43Example 25 used 0 13.8 91 120 Example 44 Example 26 used 0 13.8 95 90Example 45 Example 27 used 0 13.5 95 90 Example 46 Example 28 used 013.6 95 180 Example 47 Example 29 used 0 13.5 92 180 Example 48 Example30 used 0 13.7 95 120 Example 49 Example 31 used 0 13.3 90 120 Example50 Example 32 used 0 13.5 97 120 Example 51 Example 33 used 0 13.8 97 60Example 52 Example 34 used 0 13.7 95 60 Example 53 Example 35 used 013.2 90 120 Example 54 Example 36 used 0 13.6 95 180 Example 55 Example37 used 0 13.1 95 180 Example 56 Example 38 used 0 13.5 95 180 Example57 Example 39 used 0 13.3 95 180

[0262] TABLE 7 Acicular hematite particles washed with water afteraqueous alkaline solution treatment Average major Average minorGeometrical axial diameter axial diameter standard Aspect ratio*Examples (μm) (μm) deviation σg (−) (−) Example 40 0.132 0.0209 1.356.32 Example 41 0.138 0.0223 1.38 6.19 Example 42 0.137 0.0230 1.38 5.96Example 43 0.170 0.0301 1.35 5.65 Example 44 0.172 0.0288 1.34 5.97Example 45 0.186 0.0294 1.35 6.33 Example 46 0.203 0.0314 1.34 6.46Example 47 0.138 0.0222 1.39 6.22 Example 48 0.139 0.0243 1.40 5.72Example 49 0.210 0.0298 1.44 7.05 Example 50 0.209 0.0296 1.44 7.06Example 51 0.188 0.0285 1.34 6.60 Example 52 0.189 0.0298 1.35 6.34Example 53 0.140 0.0220 1.36 6.36 Example 54 0.140 0.0219 1.37 6.39Example 55 0.141 0.0213 1.36 6.62 Example 56 0.141 0.0211 1.36 6.68Example 57 0.143 0.0210 1.36 6.81 Acicular hematite particles washedwith water after aqueous alkaline solution treatment S_(BET) S_(TEM)S_(BET)/S_(TEM) Examples (m²/g) (m²/g) (−) Example 40 52.2 39.7 1.31Example 41 47.1 37.3 1.26 Example 42 43.7 36.3 1.21 Example 43 38.9 27.81.40 Example 44 36.1 28.9 1.25 Example 45 34.0 28.2 1.20 Example 46 35.526.4 1.35 Example 47 50.6 37.4 1.35 Example 48 52.5 34.4 1.53 Example 4936.9 27.6 1.33 Example 50 38.3 27.8 1.38 Example 51 40.8 29.0 1.41Example 52 36.8 27.8 1.32 Example 53 54.8 37.7 1.45 Example 54 56.5 37.91.49 Example 55 50.6 38.8 1.30 Example 56 50.8 39.2 1.30 Example 57 51.939.3 1.32 Acicular hematite particles washed with water after aqueousalkaline solution treatment Kind of Sintering preventive CalculatedAmount Calculated Amount Calculated Amount Examples as (wt. %) as (wt.%) as (wt. %) Example 40 Sn 14.16 — — — — Example 41 Sn 17.92 — — — —Example 42 Sn 36.65 — — SiO₂ 0.60 Example 43 Sn  1.30 — — — — Example 44Sn  3.14 — — — — Example 45 Sn  9.68 — — — — Example 46 Sn 35.60 — — — —Example 47 Sn 72.10 — — — — Example 48 Sn 90.01 — — — — Example 49 Sn53.65 — — — — Example 50 Sn 45.89 P 0.26 — — Example 51 Sn 35.68 Sb 5.16— — Example 52 Sn  9.26 Sb 1.86 — — Example 53 Sn 48.65 Sb 8.62 SiO₂1.36 Example 54 Sn 76.56 Sb 13.68  Al 0.98 Example 55 — — P 0.72 — —Example 56 — — — — SiO₂ 1.80 Example 57 — — p 0.25 SiO₂ 0.68 Acicularhematite particles washed with water after aqueous alkaline solutiontreatment Soluble sodium Volume salt Soluble sulfate pH valueresistivity Examples (ppm) (ppm) (−) (Ωcm) Example 40 108 13 9.3 3.2 ×10⁶ Example 41 135 32 9.0 1.7 × 10⁶ Example 42  87 23 9.1 1.1 × 10⁶Example 43  78 24 8.8 3.8 × 10⁷ Example 44 121 32 9.4 3.1 × 10⁷ Example45  98 46 8.6 8.2 × 10⁶ Example 46 105 48 8.9 1.0 × 10⁶ Example 47 12411 9.0 5.8 × 10⁵ Example 48 138 21 9.5 2.6 × 10⁵ Example 49  76 15 8.89.1 × 10⁵ Example 50  89 21 8.9 7.0 × 10⁵ Example 51 107 17 9.3 6.9 ×10⁵ Example 52 124 16 9.1 2.6 × 10⁶ Example 53  79  8 9.0 3.3 × 10⁵Example 54  87  8 8.9 1.3 × 10⁵ Example 55 115 15 9.2 8.9 × 10⁸ Example56 121 21 9.4 6.5 × 10⁸ Example 57  89 32 8.9 5.1 × 10⁸

[0263] TABLE 8 Wet-pulverization Heat treatment Kind of step in aqueousacicular Residue alkaline solution Comparative hematite Use or on sievepH Tempera- Time Examples particles non-use (wt. %) (−) ture (° C.)(min) Comparative Comparative used 0 — — — Example 28 Example 19Comparative Comparative used 0 11.5 93 180 Example 29 Example 20Comparative Comparative used 0 13.3 50 180 Example 30 Example 21Comparative Comparative unused 18.0 13.3 90 180 Example 31 Example 22Comparative Comparative unused 19.6 10.5 95 180 Example 32 Example 24Comparative Comparative unused 23.6 13.3 92 120 Example 33 Example 25Comparative Comparative unused 17.5 13.5 90 120 Example 34 Example 26Comparative Comparative used 0  9.5 95 120 Example 35 Example 27

[0264] TABLE 9 Acicular hematite particles washed with water afteraqueous alkaline solution treatment Average major Average minorGeometrical Comparative axial diameter axial diameter standard Aspectratio* Examples (μm) (μm) deviation σg (−) (−) Comparative 0.134 0.02061.35 6.50 Example 28 Comparative 0.134 0.0200 1.35 6.70 Example 29Comparative 0.132 0.0218 1.35 6.06 Example 30 Comparative 0.132 0.01981.35 6.67 Example 31 Comparative 0.192 0.0288 1.35 6.67 Example 32Comparative 0.192 0.0291 1.34 6.60 Example 33 Comparative 0.191 0.03021.35 6.32 Example 34 Comparative 0.185 0.0292 1.33 6.34 Example 35Acicular hematite particles washed with water after aqueous alkalinesolution treatment Comparative S_(BET) S_(TEM) S_(BET)/S_(TEM) Examples(m²/g) (m²/g) (−) Comparative 58.8 40.2 1.46 Example 28 Comparative 52.641.3 1.27 Example 29 Comparative 44.1 38.2 1.15 Example 30 Comparative69.1 41.8 1.65 Example 31 Comparative 43.7 28.7 1.52 Example 32Comparative 50.9 28.4 1.79 Example 33 Comparative 38.0 27.5 1.38 Example34 Comparative 63.8 28.4 2.24 Example 35 Acicular hematite particleswashed with water after aqueous alkaline solution treatment Kind ofSintering Specific preventive Soluble Soluble volume ComparativeCalculated Amount sodium sulfate pH value resistivity Examples as (wt.%) salt (ppm) (ppm) (−) (Ωcm) Comparative P 0.46 658 354 7.1 5.1 × 10⁸Example 28 Comparative SiO₂ 1.38 452 316 7.0 6.7 × 10⁸ Example 29Comparative SiO₂ 0.21 365 197 7.7 7.4 × 10⁸ Example 30 Comparative P0.75 312 165 7.9 8.0 × 10⁸ Example 31 Comparative SiO₂ 1.20 703 335 7.06.1 × 10⁸ Example 32 Comparative P 1.44 321 185 7.5 7.9 × 10⁸ Example 33Comparative SiO₂ 0.23 376 167 7.9 3.7 × 10⁸ Example 34 Comparative SiO₂0.04 832 349 7.1 8.6 × 10⁷ Example 35

[0265] TABLE 10 Kind of acicular Surface treatment hematite Amountparticles added treated with (calculated aqueous as each Coatingsubstance alkaline element) Calculated Amount Examples solution Kind(wt. %) as (wt. %) Example 58 Example 40 Sodium 1.3 Al 1.28 aluminateExample 59 Example 41 Water 0.5 SiO₂ 0.45 glass #3 Example 60 Example 42Aluminum 1.0 Al 0.99 sulfate Example 61 Example 43 Colloidal 1.0 SiO₂0.96 silica Example 62 Example 44 Aluminum 1.5 Al 1.46 acetate Water 0.8SiO₂ 0.77 glass #3 Example 63 Example 45 Aluminum 0.3 Al 0.30 sulfateWater 2.5 SiO₂ 2.38 glass #3 Example 64 Example 46 Sodium 5.0 Al 4.76aluminate Example 65 Example 47 Sodium 1.5 Al 1.46 aluminate Colloidal2.5 SiO₂ 2.36 silica Example 66 Example 48 Sodium 0.2 Al 0.20 aluminateExample 67 Example 49 Aluminum 10.0 Al 9.01 acetate Colloidal 0.3 SiO₂0.28 silica Example 68 Example 50 Water 3.0 SiO₂ 2.90 glass #3 Example69 Example 51 Sodium 1.8 Al 1.76 aluminate Example 70 Example 52Aluminum 0.2 Al 0.20 acetate Example 71 Example 53 Sodium 3.0 Al 2.91aluminate Example 72 Example 54 Water 1.5 SiO₂ 1.45 glass #3 Example 73Example 55 Sodium 3.0 Sn 2.90 stannate Example 74 Example 56 Stannous5.0 Sn 4.71 chloride Example 75 Example 57 Sodium 5.5 Sn 5.11 stannateAntimony 0.5 Sb 0.47 chloride

[0266] TABLE 11 Properties of acicular hematite particles washed withwater after aqueous alkaline solution treatment Average major Averageminor Geometrical axial diameter axial diameter standard Aspect ratio*Examples (μm) (μm) deviation σg (−) (−) Example 58 0.132 0.0209 1.356.32 Example 59 0.137 0.0222 1.38 6.17 Example 60 0.137 0.0231 1.38 5.93Example 61 0.170 0.0300 1.35 5.67 Example 62 0.171 0.0288 1.35 5.94Example 63 0.187 0.0294 1.35 6.36 Example 64 0.202 0.0313 1.34 6.45Example 65 0.138 0.0222 1.39 6.22 Example 66 0.139 0.0244 1.40 5.70Example 67 0.209 0.0298 1.44 7.01 Example 68 0.210 0.0296 1.44 7.09Example 69 0.188 0.0285 1.34 6.60 Example 70 0.189 0.0298 1.35 6.34Example 71 0.141 0.0221 1.36 6.38 Example 72 0.140 0.0220 1.37 6.36Example 73 0.141 0.0213 1.36 6.62 Example 74 0.141 0.0211 1.36 6.68Example 75 0.143 0.0210 1.36 6.81 Properties of acicular hematiteparticles washed with water after aqueous alkaline solution treatmentS_(BET) S_(TEM) S_(BET)/S_(TEM) Examples (m²/g) (m²/g) (−) Example 5851.9 39.7 1.31 Example 59 46.9 37.5 1.25 Example 60 43.2 36.1 1.20Example 61 38.7 27.9 1.39 Example 62 36.0 29.0 1.24 Example 63 35.3 28.21.25 Example 64 36.5 26.5 1.38 Example 65 52.3 37.4 1.40 Example 66 51.534.3 1.50 Example 67 40.0 27.7 1.45 Example 68 40.8 27.8 1.47 Example 6940.1 29.0 1.38 Example 70 36.5 27.8 1.31 Example 71 55.1 37.5 1.47Example 72 55.6 37.7 1.47 Example 73 51.0 38.8 1.31 Example 74 52.6 39.21.34 Example 75 52.6 39.3 1.34 Properties of acicular hematite particleswashed with water after aqueous alkaline solution treatment Kind ofSintering preventive Calculated Amount Calculated Amount CalculatedAmount Examples as (wt. %) as (wt. %) as (wt. %) Example 58 Sn 13.75 — —— — Example 59 Sn 17.68 — — — — Example 60 Sn 34.52 — — SiO₂ 0.58Example 61 Sn  1.28 — — — — Example 62 Sn  3.06 — — — — Example 63 Sn 9.22 — — — — Example 64 Sn 33.81 — — — — Example 65 Sn 70.12 — — — —Example 66 Sn 88.82 — — — — Example 67 Sn 48.77 — — — — Example 68 Sn44.00 P 0.25 — — Example 69 Sn 34.68 Sb 5.11 — — Example 70 Sn  8.92 Sb1.83 — — Example 71 Sn 47.32 Sb 8.58 SiO₂ 1.36 Example 72 Sn 74.82 Sb12.99  Al 0.98 Example 73 — — P 0.69 — — Example 74 — — — — SiO₂ 1.71Example 75 — — P 0.23 SiO₂ 0.64 Properties of acicular hematiteparticles washed with water after aqueous alkaline solution treatmentSoluble sodium Volume salt Soluble sulfate pH value resistivity Examples(ppm) (ppm) (−) (Ωcm) Example 58 97  9 9.3 4.1 × 10⁶ Example 59 76 139.0 1.8 × 10⁶ Example 60 65  6 9.4 1.3 × 10⁶ Example 61 85 12 8.9 5.0 ×10⁷ Example 62 56  2 9.6 4.5 × 10⁷ Example 63 123  12 9.3 1.2 × 10⁷Example 64 76 10 9.0 2.3 × 10⁶ Example 65 107   7 9.2 6.9 × 10⁵ Example66 65  5 9.6 2.5 × 10⁵ Example 67 135  34 8.8 2.4 × 10⁶ Example 68 87 129.1 9.3 × 10⁵ Example 69 54 11 9.0 8.8 × 10⁵ Example 70 46  2 9.2 2.7 ×10⁶ Example 71 68 16 9.0 6.4 × 10⁵ Example 72 100  11 8.9 2.5 × 10⁵Example 73 89 12 9.2 6.8 × 10⁸ Example 74 78  6 9.3 3.2 × 10⁸ Example 7566 11 9.0 1.1 × 10⁸

[0267] TABLE 12 Kind of high- density Heat treatment acicular Tempera-Coating substance hematite ture Time Calculated Amount Examplesparticles (° C.) (min) as (wt. %) Example 76 Example 73 400 60 Sn 2.92Example 77 Example 74 350 60 Sn 4.75 Example 78 Example 75 380 90 Sn5.32 Sb 0.49

[0268] TABLE 13 Properties of acicular hematite particles washed withwater after treatment with tin compound Geometrical Average majorAverage minor standard Aspect axial diameter axial diameter deviationratio* Examples (μm) (μm) σg (−) (—) Example 76 0.141 0.0213 1.36 6.62Example 77 0.141 0.0210 1.36 6.71 Example 78 0.143 0.0209 1.36 6.84S_(BET) S_(TEM) S_(BET)/S_(TEM) Examples (m²/g) (m²/g) (−) Example 7650.6 38.8 1.30 Example 77 51.1 39.4 1.30 Example 78 51.9 39.5 1.31 Kindof Sintering preventive Examples Calculated as Amount (wt. %) Example 76P 0.70 Example 77 SiO₂ 1.74 Example 78 P 0.24 SiO₂ 0.67 Soluble sodiumVolume salt Soluble sulfate pH value resistivity Examples (ppm) (ppm)(−) (Ωcm) Example 76 125 15 8.9 2.6 × 10⁶ Example 77 115 26 9.0 8.1 ×10⁶ Example 78  91 32 9.0 1.6 × 10⁶

[0269] TABLE 14 Production of non-magnetic coating material Non-magneticKind of coating acicular Weight ratio material hematite of particlesViscosity Examples particles to resin (−) (cP) Example 79 Example 40 5.0410 Example 80 Example 41 5.0 435 Example 81 Example 42 5.0 384 Example82 Example 43 5.0 333 Example 83 Example 44 5.0 205 Example 84 Example45 5.0 179 Example 85 Example 46 5.0 128 Example 86 Example 47 5.0 742Example 87 Example 48 5.0 896 Example 88 Example 49 5.0 205 Example 89Example 50 5.0 230 Example 90 Example 51 5.0 512 Example 91 Example 525.0 384 Example 92 Example 53 5.0 768 Example 93 Example 54 5.0 819Example 94 Example 58 5.0 384 Example 95 Example 59 5.0 384 Non-magneticundercoat layer Young's Thickness Surface modulus of coating Glossroughness (relative Examples layer (μm) (%) Ra (nm) value) Example 793.5 190 7.2 126 Example 80 3.4 186 7.5 124 Example 81 3.4 181 7.8 125Example 82 3.4 198 6.8 129 Example 83 3.3 196 6.8 127 Example 84 3.5 1947.6 133 Example 85 3.5 188 8.4 131 Example 86 3.4 185 8.8 124 Example 873.3 180 10.4  123 Example 88 3.5 187 3.0 134 Example 89 3.4 191 7.8 136Example 90 3.5 195 7.4 128 Example 91 3.4 199 7.0 131 Example 92 3.5 1858.6 124 Example 93 3.5 185 8.3 126 Example 94 3.4 185 7.0 128 Example 953.5 180 7.3 125

[0270] TABLE 15 Production of non-magnetic coating material Non-magneticKind of Weight ratio coating acicular of particles material hematite toresin Viscosity Examples particles (−) (cP) Example 96 Example 60 5.0307 Example 97 Example 61 5.0 333 Example 98 Example 62 5.0 179 Example99 Example 63 5.0 123 Example 100 Example 64 5.0 102 Example 101 Example65 5.0 512 Example 102 Example 66 5.0 768 Example 103 Example 67 5.0 179Example 104 Example 68 5.0 205 Example 105 Example 69 5.0 384 Example106 Example 70 5.0 230 Example 107 Example 71 5.0 410 Example 108Example 72 5.0 435 Example 109 Example 76 5.0 384 Example 110 Example 775.0 435 Example 111 Example 78 5.0 512 Non-magnetic undercoat layerYoung's Thickness Surface modulus of coating Gloss roughness (relativeExamples layer (μm) (%) Ra (nm) value) Example 96 3.4 191 7.3 127Example 97 3.4 205 6.8 131 Example 98 3.4 200 6.5 131 Example 99 3.3 1937.4 136 Example 100 3.4 195 7.9 134 Example 101 3.4 190 8.3 125 Example102 3.5 188 8.6 125 Example 103 3.5 193 8.8 136 Example 104 3.4 194 8.5139 Example 105 3.5 196 8.0 131 Example 106 3.4 207 6.8 135 Example 1073.3 190 7.4 127 Example 108 3.4 195 7.3 127 Example 109 3.4 195 7.5 128Example 110 3.4 198 7.2 128 Example 111 3.4 201 6.9 129

[0271] TABLE 16 Production of non-magnetic coating material Non-magneticKind of Weight ratio coating acicular of particles material Comparativehematite to resin Viscosity Examples particles (−) (cP) ComparativeComparative 5.0 25600 Example 36 Example 1 Comparative Comparative 5.0128 Example 37 Example 15 Comparative Comparative 5.0 102 Example 38Example 16 Comparative Comparative 5.0 20480 Example 39 Example 3Comparative Comparative 5.0 768 Example 40 Example 17 ComparativeComparative 5.0 563 Example 41 Example 18 Comparative Comparative 5.0435 Example 42 Example 28 Comparative Comparative 5.0 405 Example 43Example 29 Comparative Comparative 5.0 384 Example 44 Example 30Comparative Comparative 5.0 640 Example 45 Example 31 ComparativeComparative 5.0 512 Example 46 Example 23 Comparative Comparative 5.0435 Example 47 Example 32 Comparative Comparative 5.0 435 Example 48Example 33 Comparative Comparative 5.0 410 Example 49 Example 34Comparative Comparative 5.0 2560 Example 50 Example 35 Non-magneticundercoat layer Young's Thickness Surface modulus Comparative of coatingGloss roughness (relative Examples layer (μm) (%) Ra (nm) value)Comparative 3.6 68 112.0 76 Example 36 Comparative 3.5 45 181.0 58Example 37 Comparative 3.5 89 78.5 86 Example 38 Comparative 3.7 76 84.077 Example 39 Comparative 3.5 135 33.8 103 Example 40 Comparative 3.5159 23.1 105 Example 41 Comparative 3.5 170 15.9 110 Example 42Comparative 3.6 176 13.1 112 Example 43 Comparative 3.5 175 13.2 110Example 44 Comparative 3.6 165 17.0 105 Example 45 Comparative 3.7 14721.7 109 Example 46 Comparative 3.6 167 18.6 112 Example 47 Comparative3.8 171 16.2 110 Example 48 Comparative 3.5 175 14.4 114 Example 49Comparative 3.6 148 22.7 104 Example 50

[0272] TABLE 17 Magnetic recording medium using magnetic particlescontaining iron as main ingredient Weight ratio Kind of mag- Non- neticmagnetic particles undercoat Kind of magnetic particles to resinExamples layer containing iron as main ingredient (−) Example 112Example 79 [major axial diameter = 0.10 μm; 5.0 minor axial diameter =0.016 μm; Example 113 Example 80 aspect ratio = 6.3; 5.0 Example 114Example 81 Hc = 1926 Oe; 5.0 Example 115 Example 82 σs = 131.0 emu/g;5.0 Example 116 Example 83 pH value = 10.3; 5.0 Example 117 Example 84Al content = 4.11 wt. %; 5.0 Example 118 Example 85 Co content = 5.87wt. %] 5.0 Example 119 Example 86 [major axial diameter = 0.12 μm; 5.0Example 120 Example 87 minor axial diameter = 0.018 μm; 5.0 Example 121Example 88 aspect ratio = 7.0; 5.0 Example 122 Example 89 Hc = 1770 Oe;5.0 Example 123 Example 90 σs = 138.0 emu/g; 5.0 Example 124 Example 91pH value = 9.8; 5.0 Example 125 Example 92 Al content = 2.27 wt. %; 5.0Example 126 Example 93 Co content = 3.72 wt. %] 5.0 Thickness ofCoercive Surface magnetic force Br/Bm Gloss roughness Examples layer(μm) (Oe) (−) (%) Ra (nm) Example 112 1.1 2022 0.88 210 7.0 Example 1131.1 2035 0.88 206 7.0 Example 114 1.1 2041 0.88 205 7.6 Example 115 1.02087 0.88 221 6.4 Example 116 1.1 2036 0.88 223 6.3 Example 117 1.1 20260.87 212 7.0 Example 118 1.1 2016 0.88 208 7.4 Example 119 1.1 1856 0.89203 8.0 Example 120 1.2 1836 0.87 196 8.6 Example 121 1.1 1897 0.88 2007.6 Example 122 1.0 1893 0.89 211 7.4 Example 123 1.1 1834 0.89 209 7.2Example 124 1.1 1867 0.90 227 6.5 Example 125 1.2 1870 0.88 213 7.0Example 126 1.1 1867 0.88 216 7.0 Corrosion property Young's Linear Rateof modulus absorption Surface change in Rate of (relative coefficientresistivity coercive change in Examples value) (μm⁻¹) (Ω/sq) force (%)Bm (%) Example 112 131 1.23 9.6 × 10⁷ 7.9 6.9 Example 113 130 1.24 5.1 ×10⁷ 6.4 5.4 Example 114 132 1.21 9.8 × 10⁷ 5.3 5.8 Example 115 134 1.274.6 × 10⁷ 5.7 6.9 Example 116 134 1.29 4.4 × 10⁷ 7.3 6.4 Example 117 1381.31 1.8 × 10⁸ 3.7 4.8 Example 118 135 1.31 8.8 × 10⁷ 6.4 7.9 Example119 130 1.20 6.4 × 10⁶ 4.8 6.3 Example 120 131 1.20 7.8 × 10⁵ 5.3 7.2Example 121 139 1.30 8.6 × 10⁶ 8.6 7.6 Example 122 141 1.31 7.3 × 10⁶7.4 7.9 Example 123 132 1.28 1.0 × 10⁷ 5.8 6.8 Example 124 133 1.26 7.8× 10⁷ 6.7 5.2 Example 125 130 1.23 6.5 × 10⁶ 6.6 4.8 Example 126 1301.22 8.7 × 10⁵ 3.6 3.5

[0273] TABLE 18 Magnetic recording medium using magnetic particlescontaining iron as main ingredient Weight ratio Kind of mag- Non- neticmagnetic particles undercoat Kind of magnetic particles to resinExamples layer containing iron as main ingredient (−) Example 127Example 94 [major axial diameter = 0.10 μm; 5.0 minor axial diameter =0.016 μm; Example 128 Example 95 aspect ratio = 6.3; 5.0 Example 129Example 96 Hc = 1926 Oe; 5.0 Example 130 Example 97 σs = 131.0 emu/g;5.0 Example 131 Example 98 pH value = 10.3; 5.0 Example 132 Example 99Al content = 4.11 wt. %; 5.0 Example 133 Example 100 Co content = 5.87wt. %] 5.0 Example 134 Example 101 [major axial diameter = 0.12 μm; 5.0Example 135 Example 102 minor axial diameter = 0.018 μm; 5.0 Example 136Example 103 aspect ratio = 7.0; 5.0 Example 137 Example 104 Hc = 1770Oe; 5.0 Example 138 Example 105 σs = 138.0 emu/g; 5.0 Example 139Example 106 pH value = 9.8; 5.0 Example 140 Example 107 Al content =2.27 wt. %; 5.0 Example 141 Example 108 Co content = 3.72 wt. %] 5.0Example 142 Example 109 5.0 Example 143 Example 110 5.0 Example 144Example 111 5.0 Thickness of Coercive Surface magnetic force Br/Bm Glossroughness Examples layer (μm) (Oe) (−) (%) Ra (nm) Example 127 1.1 20320.89 223 6.8 Example 128 1.1 2043 0.88 215 7.0 Example 129 1.1 2036 0.89209 7.3 Example 130 1.0 2075 0.88 220 6.4 Example 131 1.1 2046 0.90 2256.2 Example 132 1.1 2050 0.89 227 6.8 Example 133 1.2 2043 0.90 219 7.2Example 134 1.2 1864 0.90 218 7.6 Example 135 1.2 1845 0.88 209 9.0Example 136 1.1 1887 0.88 210 7.4 Example 137 1.1 1895 0.90 217 7.0Example 138 1.0 1845 0.90 217 7.0 Example 139 1.1 1876 0.91 236 6.1Example 140 1.1 1881 0.88 218 6.8 Example 141 1.1 1875 0.89 223 6.7Example 142 1.2 1881 0.89 211 7.5 Example 143 1.1 1872 0.90 215 7.3Example 144 1.1 1865 0.90 216 7.0 Corrosion property Young's Linear Rateof modulus absorption Surface change in Rate of (relative coefficientresistivity coercive change in Examples value) (μm⁻¹) (Ω/sq) force (%)Bm (%) Example 127 134 1.24 9.6 × 10⁷ 4.5 4.7 Example 128 130 1.24 5.3 ×10⁷ 5.4 4.9 Example 129 133 1.23 1.0 × 10⁸ 4.5 4.7 Example 130 136 1.274.6 × 10⁸ 5.5 6.6 Example 131 137 1.29 4.8 × 10⁸ 6.8 6.3 Example 132 1411.32 2.5 × 10⁸ 3.2 3.8 Example 133 138 1.31 9.9 × 10⁷ 3.6 4.7 Example134 130 1.21 7.2 × 10⁶ 3.7 5.1 Example 135 131 1.21 8.5 × 10⁵ 5.0 6.4Example 136 139 1.30 9.0 × 10⁶ 6.8 6.5 Example 137 144 1.30 8.7 × 10⁶6.1 6.0 Example 138 132 1.28 2.1 × 10⁷ 4.7 5.5 Example 139 140 1.27 8.4× 10⁷ 5.4 4.1 Example 140 134 1.23 8.0 × 10⁶ 2.9 3.7 Example 141 1351.23 9.8 × 10⁵ 2.8 2.9 Example 142 133 1.23 3.6 × 10⁸ 7.2 6.8 Example143 134 1.23 2.1 × 10⁸ 8.1 6.8 Example 144 135 1.24 9.6 × 10⁷ 6.0 5.2

[0274] TABLE 19 Magnetic recording medium using magnetic particlescontaining iron as main ingredient Weight ratio Kind of mag- Non- neticmagnetic particles Comparative undercoat Kind of magnetic particles toresin Examples layer containing iron as main ingredient (−) ComparativeComparative [major axial diameter = 0.10 μm; 5.0 Example 51 Example 36minor axial diameter = 0.016 μm; Comparative Comparative aspect ratio =6.3; 5.0 Example 52 Example 37 Hc = 1926 Oe; Comparative Comparative σs= 131.0 emu/g; 5.0 Example 53 Example 38 pH value = 10.3; ComparativeComparative Al content = 4.11 wt. %; 5.0 Example 54 Example 39 Cocontent = 5.87 wt. %] Comparative Comparative 5.0 Example 55 Example 40Comparative Comparative 5.0 Example 56 Example 41 ComparativeComparative 5.0 Example 57 Example 42 Comparative Comparative 5.0Example 58 Example 43 Comparative Comparative 5.0 Example 59 Example 44Comparative Comparative 5.0 Example 60 Example 45 ComparativeComparative 5.0 Example 61 Example 46 Comparative Comparative 5.0Example 62 Example 47 Comparative Comparative 5.0 Example 63 Example 48Comparative Comparative 5.0 Example 64 Example 49 ComparativeComparative 5.0 Example 65 Example 50 Thickness of Coercive SurfaceComparative magnetic force Br/Bm Gloss roughness Examples layer (μm)(Oe) (−) (%) Ra (nm) Comparative 1.3 1976 0.77 123 76.5 Example 51Comparative 1.2 1987 0.81 132 68.3 Example 52 Comparative 1.2 1980 0.82165 31.6 Example 53 Comparative 1.2 1991 0.78 154 46.2 Example 54Comparative 1.2 2001 0.83 175 17.9 Example 55 Comparative 1.3 2010 0.84187 13.2 Example 56 Comparative 1.1 2018 0.86 191 11.8 Example 57Comparative 1.3 2028 0.87 194 11.6 Example 58 Comparative 1.1 1999 0.86190 12.6 Example 59 Comparative 1.2 2007 0.84 177 14.7 Example 60Comparative 1.1 1989 0.84 165 21.6 Example 61 Comparative 1.0 2011 0.85181 13.8 Example 62 Comparative 1.3 1997 0.85 188 12.1 Example 63Comparative 1.3 2023 0.85 188 11.9 Example 64 Comparative 1.2 2017 0.84165 23.8 Example 65 Corrosion property Young's Linear Rate of modulusabsorption Surface change in Rate of Comparative (relative coefficientresistivity coercive change in Examples value) (μm⁻¹) (Ω/sq) force (%)Bm (%) Comparative 90 0.84 8.9 × 10⁸ 27.5 25.4 Example 51 Comparative 730.90 8.3 × 10⁸ 38.9 31.1 Example 52 Comparative 96 0.95 9.6 × 10⁸ 49.836.8 Example 53 Comparative 89 0.99 1.1 × 10⁹ 28.2 23.7 Example 54Comparative 113 1.09 1.3 × 10⁹ 46.9 39.7 Example 55 Comparative 116 1.138.7 × 10⁸ 37.6 33.3 Example 56 Comparative 121 1.15 2.3 × 10⁹ 17.1 15.8Example 57 Comparative 116 1.15 1.0 × 10¹⁰ 14.2 13.8 Example 58Comparative 121 1.14 7.2 × 10⁹ 16.4 16.7 Example 59 Comparative 121 1.173.6 × 10⁹ 16.3 19.0 Example 60 Comparative 116 1.19 1.8 × 10⁹ 37.4 31.6Example 61 Comparative 121 1.19 4.1 × 10⁹ 18.9 23.1 Example 62Comparative 119 1.17 7.2 × 10⁹ 15.7 18.5 Example 63 Comparative 123 1.186.5 × 10⁹ 18.5 16.9 Example 64 Comparative 118 1.06 9.6 × 10⁸ 23.7 24.8Example 65

What is claimed is:
 1. High-density acicular hematite particlescomprising acicular hematite particles and a coat comprising an oxide oftin or oxides of tin and antimony, formed on at least a part of surfacesof said acicular hematite particles; and having an average major axialdiameter of not more than 0.3 μm, a pH value of not less than 8, asoluble sodium salt content of not more than 300 ppm, calculated as Na,and a soluble sulfate content of not more than 150 ppm, calculated asSO₄.
 2. High-density acicular hematite particles according to claim 1 ,which further comprise a coat comprising at least one compound selectedfrom the group consisting of a hydroxide of aluminum, an oxide ofaluminum, a hydroxide of silicon and an oxide of silicon, formed on atleast a part of surfaces of said high-density acicular hematiteparticles.
 3. High-density acicular hematite particles according toclaim 1 , wherein the amount of said oxide of tin is 0.5 to 500% byweight, calculated as Sn, based of the weight of said acicular hematiteparticles.
 4. High-density acicular hematite particles according toclaim 1 , wherein the amount of said oxide of antimony is 0.05 to 50% byweight, calculated as Sb, based of the weight of said acicular hematiteparticles.
 5. High-density acicular hematite particles according toclaim 1 , wherein the weight ratio of tin to antimony is 20:1 to 1:1. 6.High-density acicular hematite particles according to claim 1 , whichfurther have an aspect ratio (average major axial diameter:average minoraxial diameter) of not less than 2:1.
 7. High-density acicular hematiteparticles according to claim 1 , which further have a degree ofdensification (S_(BET)/S_(TEM)) of 0.5 to 2.5, wherein S_(BET)represents a specific surface area measured by a BET method, and S_(TEM)represents a surface area calculated from values of major axial diameterand minor axial diameter obtained by measurement of particles onelectron microscope photograph.
 8. High-density acicular hematiteparticles according to claim 1 , which further have a particle sizedistribution of major axial diameter represented by a geometricalstandard deviation of not more than 1.50.
 9. High-density acicularhematite particles according to claim 1 , which further have a BETspecific surface area of not less than 35 m²/g.
 10. High-densityacicular hematite particles according to claim 1 , which further have avolume resistivity of 10³ to 5×10⁷ Ωcm.
 11. High-density acicularhematite particles according to claim 2 , wherein the amount of saidhydroxide of aluminum or said oxide of aluminum is 0.01 to 50% byweight, calculated as Al, based of the weight of said acicular hematiteparticles.
 12. High-density acicular hematite particles according toclaim 2 , wherein the amount of said hydroxide of silicon or said oxideof silicon is 0.01 to 50% by weight, calculated as SiO₂, based of theweight of said acicular hematite particles.
 13. An undercoat layercomprising the high-density acicular hematite particles set forth inclaim 1 and a binder resin, formed on a non-magnetic substrate.
 14. Anundercoat layer according to claim 13 , wherein the amount of saidhigh-density acicular hematite particles is 5 to 2,000 parts by weightbased on 100 parts by weight of said binder resin.
 15. An undercoatlayer according to claim 13 , which have a thickness of 0.2 to 10.0 μm.16. An undercoat layer according to claim 13 , which further have agloss of 180 to 280% and a surface roughness of 2.0 to 13.0 nm.
 17. Amagnetic recording medium comprising: a non-magnetic substrate; anon-magnetic undercoat layer comprising the high-density acicularhematite particles set forth in claim 1 and a binder resin, formed onsaid non-magnetic substrate; and a magnetic recording layer comprisingmagnetic particles containing iron as a main ingredient and a binderresin, formed on said non-magnetic undercoat layer.
 18. A magneticrecording medium according to claim 17 , wherein said high-densityacicular hematite particles are acicular hematite particles set forth inany of claims 2 to 12 .
 19. A magnetic recording medium according toclaim 17 , wherein said magnetic particles containing iron as a mainingredient comprise 50 to 99% by weight of iron, 0.05 to 10% by weightof aluminum, and at least one selected from the group consisting of Co,Ni, P, Si, Zn, Ti, Cu, B, Nd, La and Y.
 20. A magnetic recording mediumaccording to claim 17 , wherein said magnetic particles containing ironas a main ingredient comprise 50 to 99% by weight of iron, 0.05 to 10%by weight of aluminum, and at least one rare earth metal selected fromthe group consisting of Nd, La and Y.
 21. A magnetic recording mediumaccording to claim 17 , wherein said magnetic particles containing ironas a main ingredient have an average major axial diameter of 0.01 to0.50 μm, an average minor axial diameter of 0.0007 to 0.17 μm, an aspectratio of not less than 3:1, a resin adsorptivity of not less than 65%, acoercive force of 1200 to 3200 Oe, and a saturation magnetization of 100to 170 emu/g.
 22. A magnetic recording medium according to claim 17 ,which further have an coercive force of 900 to 3500 Oe, a squareness of0.85 to 0.95, a gloss of 195 to 300%, a surface roughness of not morethan 11.0 nm, a linear adsorption coefficient of 1.10 to 2.00 μm⁻¹, asurface resistivity of 10⁴ to 5×10⁸ Ω/sq.
 23. A magnetic recordingmedium according to claim 17 , which further have a percentage of changein said coercive force of not more than 10.0%, and a percentage ofchange in said saturation magnetization flux of not more than 10.0%. 24.A process for producing high-density acicular hematite particles setforth in claim 1 , comprising: heat-dehydrating acicular goethiteparticles coated with a hydroxide of tin to obtain low-density acicularhematite particles; heat-treating said low-density acicular hematiteparticles at a temperature of not less than 550° C. to obtainhigh-density acicular hematite particles coated with an oxide of tin;wet-pulverizing a slurry containing said high-density acicular hematiteparticles; adjusting the pH value of said slurry to not less than 13;heat-treating said slurry at a temperature of not less than 80° C.; andfiltering said slurry to separate high-density acicular hematiteparticles therefrom, followed by washing with water and drying.
 25. Aprocess for producing high-density acicular hematite particles set forthin claim 1 , comprising: wet-pulverizing a slurry containinghigh-density acicular hematite particles obtained by heat-treating at atemperature of not less than 550° C. low-density acicular hematiteparticles produced by heat-dehydrating acicular goethite particlescoated with a sintering preventive agent; adjusting the pH value of saidslurry to not less than 13; heat-treating said slurry at a temperatureof not less than 80° C.; and filtering said slurry to separatehigh-density acicular hematite particles therefrom, followed by washingwith water and drying; treating the obtained high-density acicularhematite particles with an aqueous solution containing a tin compound toobtain high-density acicular hematite particles coated with a hydroxideof tin; and heat-treating said high-density acicular hematite particlescoated with a hydroxide of tin at a temperature of not less than 300° C.26. A process for producing high-density acicular hematite particles setforth in claim 1 , comprising: heat-dehydrating acicular goethiteparticles coated with hydroxides of tin and antimony to obtainlow-density acicular hematite particles; heat-treating said low-densityacicular hematite particles at a temperature of not less than 550° C. toobtain high-density acicular hematite particles coated with oxides oftin and antimony; wet-pulverizing a slurry containing said high-densityacicular hematite particles; adjusting the pH value of said slurry tonot less than 13; heat-treating said slurry at a temperature of not lessthan 80° C.; and filtering said slurry to separate high-density acicularhematite particles therefrom, followed by washing with water and drying.27. A process for producing high-density acicular hematite particles setforth in claim 1 , comprising: wet-pulverizing a slurry containinghigh-density acicular hematite particles obtained by heat-treating at atemperature of not less than 550° C. low-density acicular hematiteparticles produced by heat-dehydrating acicular goethite particlescoated with a sintering preventive agent; adjusting the pH value of saidslurry to not less than 13; heat-treating said slurry at a temperatureof not less than 80° C.; and filtering said slurry to separatehigh-density acicular hematite particles therefrom, followed by washingwith water and drying; treating the obtained high-density acicularhematite particles with an aqueous solution containing a tin compoundand an antimony compound to obtain high-density acicular hematiteparticles coated with hydroxides of tin and antimony; and heat-treatingsaid high-density acicular hematite particles coated with hydroxides oftin and antimony at a temperature of not less than 300° C.